@Acro lost a huge post might come back to this later, but TLDR: I'm not talking about the shallowness of the character herself I'm talking about the shallowness of the writer and how she herself is characterized
which is "what if an adult was actually five ****ing years old and entangled by a power play for world governance by a misanthropic teenager"
Her personality is what it is for the purpose of panderin' and nothin' else. Your shia lebouf example is valid but not exactly what I mean here. In 75 years if the "Shia Lewhatever how do you spell it Persona" becomes an overused and shallowly implemented stereotype, then using it would be pandering. Shia's roles being almost indistinguishable however is not pandering, that is typecasting. Putting Shia in Indiana ****ing Jones (seriously what in the damn ****) in the first place, however, was the pandering in question. ...and putting him in most things, I guess, he was serviceable in Lawless but you get me. The producers even admitted to it funny enough (though they of course phrased it in a way that makes it sound far less stupid than it was, something about "bringing the character of Indy to younger fans"... or some ****). Basically they were forthright about "he's only in this movie because we don't trust young people to like a good character because he's too OLD and young people are *******". Live Free or Die Hard did the same thing with Justin Long's character, although I actually thought they did that pretty well given that the dichotomy was well-used to display the generational difference between the characters and made McClane more of a fish out of water than ever. Too bad the movie was so toothless, which is actually another less forgivable instance of pandering since they forced the content of the film to be more explosion-ridden and technophilic to appeal to young people and then chopped the violence down to nothing to secure a PG-13 so that all the lil babbies they were trying to appeal to could even see the movie.
(For those inevitably wondering about my opinion, I have no intention to see Die Hard 5 given that they are recycling THE EXACT SAME PANDERING TRICK by giving McClane another handsome 20something sidekick, making it "bigger and better" (aka more explosive and more geared to the ADD generation and worse), and even some more pandering by adding all this espionage bull**** to basically force the movie to be a Bourne movie instead of a Die Hard movie...... but now I"m rambling and way off my original point)
so yeah you see all the functions you mention about both (1) Misa's character motivations/fixation on Light/etc and (2) varying purposes in the plot...... NONE of them are served better by writing her as an overactive five year old that ate a bunch of candy. None. Not one. None at all.
Everything about her personality is incongruous to the tone of a show that with the exception of herself, somewhat-Near, and mostly-Mello, has well-drawn characters that I could see existing in the real world. Her personality doesn't serve her role in the work, at all. In fact the grating simpering idiocy of it drags the show's quality and maturity down significantly in every scene.
Amane Misa could very easily be reworked into an actual ****ing character with all the same motivations, roles in the plot, you name it, and some actual character depth. Or at the very least not portrayed in a manner that is quite frankly insulting to my ****ing intelligence. There's a difference between a character's role in a work's story or core conception as a person within the work's world and how they are actually characterized and portrayed within the writing/performance/etc. The former is fine, and that's what you're using to justify the latter. The latter is stupid and doesn't work, and there is no excuse for a major character to be so clearly designed from the ground up to pander to the people that like cutesy kawaii desu ne ^____^ characters.
She's airheaded, she loves Light/Kira unconditionally, she is singular in her fixation, she's a superstar celebrity everybody loves, she's shallow. All of that can work. The way the writer and the anime people chose to do that..... doesn't. It's awful.