Marion, I think the shows are meant to be humorous because of the absurdity. Knowing the female singers are really male is like a big practical joke that everyone's in on. Why is someone who does impressions funny? The drag queens do an impression of an entire sex instead of an individual. I haven't seen the movie you mentioned yet, but maybe the characters' act was supposed to be not-so-good?

The genre's been around for quite a while, although I don't know when it actually started. For other movies involving this type of entertainment as a central theme, see "La Cage aux Folles," and "Victor/Victoria." I think "Cabaret" had a bit of cross-dressing in it, too.