A lot of good points here.
It's worth noting that she never sees Garp sympathetically, and in a way, always sees him at his worst.
Children form grudges over things we take for granted. I kept thinking in the first scene that maybe Pooh had a crush on Garp but Garp preferred her sister, so she resented him and her affection turned to hate. Or, even without the material from the book, she sees him as taking her sister away -- perhaps she feels neglected when her sister wants to play with the boys and not with her anymore.
So we've got:
1) anger toward Garp over her sister
2) biting off a piece of her dog's ear
3) leading one girl on but then trying to sleep with another
4) writing an anti-Ellen Jamesian book, and
5) sneaking into his mother's memorial service
Add it all up and her view of Garp is composed entirely of negatives.
My only foible with the movie is the seeming inability to unite the sexes and calm the severity, but maybe the film is saying that once extreme views take root, they can't be undone, and the end result can only be violence.