I think people simply complain about her because of the fact that she takes so much air time, which is only expected with her being the main character.
That is a common basis for complaint, nonetheless, though. (I admit, I've complained about it more than once.)
Well... personally, I don't like her.
Mainly because of the amount of air-time she has received (and thus, taken from other characters that were never given the depth they deserved), and because of the idea that she is a character that appeals to a certain audience, and I'm not part of that crowd.
I also think they try too hard to make her character one that can be related to moreso than the others (her being the regular "Earth girl" with a kind personality...) That is, despite the fact that she has everything going for her and has a nearly "perfect" life... (Ex: Perfect "Prince Charming" boyfriend, friends that support her no matter what, parents that support her no matter what, an interesting and mysterious past, the greatest power in the realms, etc. etc.)
In a way, I think that just defeats the attempt of making her more relatable, but that's just me.
Well... That rant above this sentence, needless to say, is simply my opinion.
Please note that I don't
hate her, though (A, it's a cartoon, B, my little sister would be terribly angry.)