Well, since the novel, the physical description of Eponine is that she's rather pretty. But, it's true, rules can be broken. We can't expect to find Lea Salonga everywhere we go. I don't think it's THAT important to Eponine to be pretty anyway.
HOWEVER, you may find me a little close-minded for saying this, but Eponine is dirt-poor, living on the streets, and starving. She would have to be at least a little bit on the slim side...does that make sense?
Sometimes it's needed, sometimes it's not.
Do I think there should be a "pretty" sterotype? Well, what I think is pretty, others think is ugly. What a producer/director might see in a girl they cast, I may go "What?? well..geez..she's not very attractive", and to them she may be the most beautiful girl on the stage.
It's tough.
I mean, Denzel Washington? Gun to my head..of course.