I couldn't agree with you more. BUT I will say this: I do believe she would be happy in her straight haired psychotic state. I actually find "Cupcakes" more creepier with the happy Pinkamena. I only believe it was written with her curly hair because it was written before the episodes "Party of one" and "Cutie Mark Chronicles. My theory is that Pinkie has struggled with these issues all her life and can't control them. I believe that she only becomes psychotic when Pinkamena takes over (kind of like a severe multiple personality thing). I believe she still is happy and stuff but a creepy happy, kind of like how she was in "Party of One". Since then, I even made my Cupcakes cosplay basing off straight haired Pinkamena.

i tend to disagree with the many radical interpretations of 'Pinkimena' while we know that Pinkie Pie turned a tad detached from her normal state of reason in the episode 'Party of One' i do not believe this secondary personality of hers is perversely and/or psychopathically homicidal,
(as the disturbing twitter forum; "Ask Pinkemena"may suggest.) in any way.
merely, as portrayed in the episode of her appearance, she is just slightly detached from reality in believing that her friends didn't like her anymore. thus making her depressed. it is true that when the episode showed her twitching involuntarily,
(with a split-second colored splash background.)
it was showing her having symptoms of a mental break down. however, there is no evidence that her behavior might be shown as either self destructive, or homicidal. as one could clearly see in the episode, she constructs make-shift 'party guests' to replace the friends she supposingly lost. thus creating a new medium of emotional support for herself.
now, one may add the point that her constructs might be potentially destructive to her remaining psyche, as the show depicts her constructs talking to her, telling her to 'forget her former friends.'
however, i believe they aren't innately destructive. even if her psychopathic delusions were telling her to do something, usually destructive to herself or others in most cases, but instead her delusional construct did nothing more that just tell her that she needed to
'forget about her friends', and that
'they don't deserve this party.'
this behavior does not suggest that she would have turned homicidal in any way. really what she might have done is simply go on partying with her delusionally constructed guests and after, remained by herself and her lizard pet from then on. now assuming that she'd attempt to become more sociable again, she would seek out new friends to hang out and party with. and try to avoid her former friends. or, if her schizophrenia persisted, stay inside and construct new 'party guests' for herself.
no "cupcakes", no homicidal tendency's at all.
just more self imposed isolation, bouts of depression, and more delusionally constructed 'party guests' which, as time goes by, might become more elaborate than just a pile of rocks or a bucket of turnips, perhaps life-sized puppet versions of her friends, no doubt.
and lastly, Pinkie Pie becoming homicidal, would never happen. regardless of an advancing psychophrenia.
her base nature is she wants to make people happy with fun parties. and so she would never consider hurting other ponies. perhaps.

if you have any counter arguments, please PM me.

Never mind; I just realized the upload date of that is later than the upload date of this.
Sorry.
_{Unless that really is the original artist, just on another account. idk}

Image if you still need it: [link]

I'm a bit late to the party, but thought I would join in.

I'll start by saying that I am not a psychologist but that I do have a lot of experience with abnormal psychology.

First of all, MPD is now known as DID. It was known as MPD under the old DSM classification but the current one calls it DID. The difference between these two being a lot to do with the redefining of the word "personality." (Which, I believe you addressed in one of your response to another person.) DID is part of a spectrum of disorders called Dissociation. On one end of the spectrum you've Complex Post-Traumatic Stress Disorder and Conversion Disorders, while on the more severe end of the scale you have Dissociative Disorder: Not Otherwise Specified (consider this one step down from DID - there are less defined alters if they are there and/or fragments instead of full-blown alters) and Dissociative Identity Disorder (where there are full-blown alters and significant time loss).

Dissociative Disorders are seen as a coping mechanism and a defense mechanism. DID is attributed to repeated and/or prolonged trauma as a child. We know that Pinkie had a dull childhood on the rockfarm, but was it necessarily traumatizing? I think it's possible to argue both ways. One could argue that a dull childhood does not imply a traumatizing childhood, or that one could argue that rather than a dull childhood Pinkie had a childhood without joy which could potentially be traumatizing given how open that is to interpretation. Similarly in the series, we saw Pinkie "switch" to Pinkamena in a time where she felt deserted and then defensive. We don't know how old Pinkie is, but since she's living and working as an adult would (as are all of her friends), I think it's safe to say that Pinkie is probably in her teens at the youngest. DID is something that is developed as a child when the sense of self has not yet been cemented. Therefore Pinkie would not be showing her disorder(s) for the first time but rather be showing something symptomatic of long-term abuse. Pinkie also seems perfectly fine later on(where we have no way of knowing if she has experienced memory loss) which could be indicative of a switch back.

I think one of the strongest arguments for Pinkie having DID is actually the sonic rainboom itself. Alters come out due to specific triggers and/or to deal with certain instances. For example, in a DID system there might be an alter who deals with emotional pain and would come out when the person is being yelled at. In this case, the sonic rainboom is the trigger to release the happier and sillier Pinkie who overtakes Pinkamena who then radically changes Pinkamena's life by partying.

Now, the argument against DID: Pinkie has a continuous memory that doesn't imply or show time lapses. It is entirely possible that Pinkie herself is amnestic for her loss of time, but she remembers her life prior to the sonic rainboom. If she had DID then these memories would seemingly belong to Pinkamena. Pinkie Pie's life would effectively start after seeing the sonic rainboom and she would just have a blank for the first however many years of her life.

Alternatively, she COULD have had DID but integrated with Pinkamena to result in a single personality which, when under stress, could plausibly splinter off to have Pinkamena revisit Pinkie's life either permanently or temporarily. She could also be aware of Pinkamena and have shared some of her memories because of this. Finally, one could explain a lack of dissociative amnesia in Pinkie Pie if she and Pinkamena were co-conscious which means that they're able to be present at the same time.

As for schizophrenia, one could certainly make an argument for this as well. For one thing, Pinkie is the only pony of the group who has "caretakers." This might just be because they had a room for rent and a job, but it bears keeping in mind. Looking at the different types of schizophrenia, childhood schizophrenia seems the most likely culprit for Pinkie. I say this because we see Pinkie's first "episode" present itself in her childhood and she continues to get stranger from there. I also think childhood schizophrenia is more likely because the onset of adult schizophrenia happens around 18-25 and while it's not out of the realm of possibility since she could fall into that age group, it wouldn't explain her childhood episode after sonic rainboom.

Coming back to the sonic rainboom, that seems to have been the tipping point for Pinkie's first episode. It could also just be when her symptoms became more apparent. Pinkie Pie does truly meet a lot of the qualifications for schizophrenia, but socially she does not. I think the one thing that clinches it for me is that she does not suffer from schizophrenia is her voice. One of the most common features of schizophrenia is a lack of affect - or a monotone speaking voice. Neither Pinkie nor Pinkamena do this. Pinkie is incredibly emotive (both vocal and physically) and while Pinkamena's body language may not be emotive of anything other than negativity and/or anger, she emphasizes words in a manner that does show emotion.

Personally, I don't think either schizophrenia or DID are perfect matches for what we see in Pinkie Pie, but it sure is fun to discuss.

or if you want to describe, happy Cupcakes Pinkie in a nutshell...

Baby Firefly from House of 1000 Corpses, dead on

Wow, really good take here. I really for the most part agree with the way her different personalities would affect a situation where she committed murder. I really like to see her psychotic/homicidal personality as being separate from her Pinkie/Pinkamena personalities. In Cupcakes, she could have shifted from her mental state where she only wants to make others happy to her state where she only wants to murder and cause pain to others when she tortured or killed anypony. When in this state, she could retain all of her happy, excited characteristics and also perfectly feign her sweet, friendly nature to confront her victims before taking them to her lair. When Rainbow Dash passed out for the first time in the fic, it obviously made Pinkie very upset, possibly lonely which could have triggered her depressed, angsty personality which she carried with her the rest of the time she spent mutilating Dash. I really like to think that her kind and sadistic personalities are finely intertwined to the point where she almost has control over which one she takes on at a time.

Damn, I feel like I've looked way to into this myself x.x Heh, I just fell in love with evil/psycho Pinkie to the point where I wanted to go crazy with the whole idea :3

Interesting analysis! In my headcanon, I usually give Pinkie MPD/DID (the better to ship her with herself?), so it's nice to know I might have some basis in canon, haha.