Because I like to believe she's the one (female) superhero that can be a dip. Afterall, she is in the Super family. It's practically a by-law in the state of New York that all SuperBeings have to be class-A dorks. Even the horse.
Edit: I forgot to add that I drew this because there's a drawing meme going around asking folks to draw SuperGirl (ala last year's BatGirl frenzy) and I'm a sucker for pre-fab themes.