Rich in Dollars, Poor in Sense: Regina's a specific example. She could rattle off several mostly-illegal weight loss drugs (most of which either were expensive before being banned or would be expensive to ship from countries where they're still legal). Yet, she didn't know enough about health and nutrition to know whether or not butter is a carb. This is most likely a jab at the stereotype of American teenage girls wanting to get thin or in shape the fast and unhealthy way than through actual healthy living.
What Could Have Been: Gretchen was originally meant to be a plain and frumpy girl whom Regina only kept around because her parents were so rich. Naturally this trait was thrown out completely when Lacey Chabert was cast. The script, however, still has a few places where Regina claims no one finds Gretchen attractive.
Advertised Extra: Karen doesn't have that large of a part (especially compared to the other three plastics) and has little impact on the plot, but is featured in all of the advertisements because she is a Plastic and thus one of the Mean Girls.
Blondes are Evil: Subverted; she's pretty nice mostly and is easily the least malicious of the Plastics (Cady included.)
Informed Ability: Apparently, her breasts can tell when it's going to rain. Or at least, when it's already raining. And even then, she's not completely sure, even if she's actually OUTSIDE in the rain.
The Pollyanna: As well as being the least malicious Plastic she's easily the happiest too, lacking Regina's rage, Gretchen's crippling neurose, or Cady's need to fit in. She's even aware of (and comfortable with) the knowledge she's not very bright.
"...And evil takes a human form in Regina George. Ah, now don't be fooled. Because she may seem like your typical selfish, backstabbing, slut-faced ho-bag, but in reality, she is so much more than that."
Ambiguously Gay: Is Janis Ian a lesbian? Damn hard to say. It's proven she's not a lesbian when she starts dating Kevin at the end of the movie.
Important Haircut: Offscreen. After Regina spread the rumor in middle school that Janis was a lesbian, Janis dropped out of school for the rest of the year; when she returned in the fall, "all her hair was cut off, and she was totally weird."
Karma Houdini: Janis, who, despite influencing and encouraging Cady to join the Plastics specifically to damage Regina, gets zero comeuppance when she reveals it to the entire crowd of girls following the revelation of the Burn Book.
Manipulative Bitch: Janis convinces Cady to join the Plastics just so she can information from them.