Some people don't like her because of her music. Others don't like her because of how she always has boyfriends and breaks up with them right after. Also, some people don't like her because she is mainstream. I actually know a few people who hate her just for that reason.
I don't have anything "against her". I don't care for the music, but I also find it disturbing she had had countless public relationships and doesn't seem to respect the other person enough that she feels the need to make another million dollars off of them in the form of a new "song"