Nasty girl (not Amber)

Luanne is the one who danced the mashed potato and puffed her cheeks out at the beginning. I assumed she was just being childish and didn't really dislike Tracy. I never noticed her agreeing with Amber about the roaches though. I'll have to look out for that next time I re-watch. My guess would be that all the dancers (save for Amber and Link) were mere background characters with no character development and therefore John Waters saw no need for consistency in there behaviors and actions.

I think most of the council members are initially judgemental of Tracey's weight - You can even hear one of them mutter "oink, oink" as she steps forward at the try-outs.
As for the scene where Lou-Anne agrees with Amber regarding the cockroaches - This can be explained by one of the movies cut scenes. Originally there was a shot of cockroaches crawling about in Tracey's hair, but John Waters ultimately decided to remove it. Ricky Lake has talked about how it was the most difficult scene for her to film.
So neither Lou-Anne or Amber are being bitchy in that scene - They really did see roaches! If you watch it again you can see the initial reaction on their faces.