She isn't. She has no chest, she's short and thin, and in game she's dumb and annoying.
If there's anything she has going for her, it's that her attire is hyper girly, and being browns are stereotypically more animal when it comes to sex. I can easily imagine Shauna taking anal, or being dicked behind a barn while still wearing her shirt. It's harder to imagine that with Serena, Diantha, or Viola willing to do that.
