I don't see why her reaction would necessarily be unrealistic or insincere, though. If I think of how I would react if somebody told me they hated a particular item of clothing I was wearing (ok, that's a bad example because I could not care less about what I wear, but say it was a book I was reading) I'd say it would depend entirely on who said it. And how they said it. And the context in which it was said.

Besides, there's a considerable difference between "I hate those trousers you are wearing" and "I hate your looks" or even "I hate the way you dress in general" (which could be interpreted to mean "You have no sense of style".)