She gets people hot and bothered - both positively and negatively - because she provokes reaction. Pure and simple.
That makes some dislike her because she's not easy to swallow. They'd prefer popstars to be inoffensive and exist purely to give them something to sing along or dance to, not think. Not saying all that she does is profound, but she does invite analysis and opinion with everything from her outfits to her videos. Some find that engaging, others find that perplexing.
Beyond that though, I guess it's a mixture of people who genuinely just don't like her and what she does, people who like to zag when others zig, people who are pressed and people who don't want to like her or rather, present that they don't, so focus on trivial reasons (i.e. disliking her stans, that she's attention seeking etc).
