It's not just that she's a stalker, but it's that everything she does is overtly creepy obsessiveness. If you find that kind of behavior cute, more power to ya, but that is most definitely not my cup of tea. Now, I'm fine with bad and unlikable characters existing in stories, obviously, but I dislike when mangas just kinda ignore how bad they are and let them get away with it, over and over.
If she was, at least, punished regularly for her borderline (if not actual) criminal behavior, I think she would be less hated as a character.