A preppy girl (usually about fifteen or sixteen) who acts like a dumbass around everyone and creates stereotypes for other girls. Stereo girls do very stupid things, rave about stupid things, and think stupid things. Eventually, if she exposes people to her stupidity, they will begin to believe all girls act like she does, therefore creating a stereotype for girls.
Guy: What's that girl doing over there?
Girl: Raving about Twilight.
Guy: Do all girls act like this?
Girl: No, she's just a stereo girl.
Guy: Oh, good.
Girl 1: God I hate Raina.
Girl 2: Why do you hate Raina?
Girl 1: She's a stereo girl and now everyone in my science class thinks I'm obsessed with Justin Bieber.
Girl 2: Aw man, I'm sorry.