I must agree that Kirsten Dunst is unremarkable when considered as a specimen of feminine beauty. Were she not famous, I think that most people, upon ecountering her in a shopping mall, would regard her as merely attractive but not extrordinarily so.
You must understand that the casting director was not allowed to cast the most beautiful, most Mary-Jane-like actress that she could find. She was constrained to cast from a tiny pool of actresses (of the right age) who have suitably distinguished themselves in past movies or TV. Given that constraint, Kirsten Dunst doesn't look like such a bad choice.
Regarding her breasts, although they're nothing to write home about, they were well used in this film. Not only was there the wet t-shirt scene and numerous low-cut blouse scenes, there was also a jiggle scene -- the one in the school cafeteria, in which she was bouncingly strutting towards a patch of slick floor on which she slips.