I think its a fair question. The girls in this movie are supposed to be a sorority, which would basically be young women of college age--aka girls at their peak hotness wise. This is a movie, so you would expect the girls would be pretty smoking hot. You wouldn't expect to find many plain Janes in a movie like this--yet here they are!
Some might view that as sexist, but then ask yourself if they cast an average looking Joe as Superman wouldn't you scratch your head? Superman is supposed to look like us, yet you never see a balding forty five year old with a pot belly as the man of steel.
Movies are entertainment. All the girls should be hot, and the dudes should look like movie stars, not smucks. It is entertainment, and producers should realize people don't go to movies to look at actors that look "true to life."
Its like those people who whine that the food in McDonalds isn't healthy enough for them. No one goes to McDonalds to eat brussell sprouts. It's fast food for a reason.
Kappa Nu are supposed to be plain Janes, not smoking hot Barbies. That's the whole point of the group. Phi Lamda is the sorority with the smoking hot Barbies. Kappa Nu is the alternative/punk sorority with the plain Janes.
Also, the fact that this thread is focusing on the girls and ignoring average-looking guys like Seth Rogan, that shows double-standards, and yes, sexism.