No, the reason why she's so popular is because of:
1. The voice-actress and her portrayal.
2. Her visual design.
She is NOT popular simply because she's one-dimensionally evil. Heck, even people who like the movie frequently say "her motivation sucks and she's not really all that interesting as a character, but the actress is great and she looks cool/has cool powers." And the simple fact is that you have to flesh her out if she's going to be the protagonist of a two-hour+ long movie (as opposed to a ninety-minute animated one where she only appears in certain spots).