I see a lot of pretty women play DUMB too. If enough guys encounter easy girls who act like bubbleheads, what's a guy supposed to think?
There you go, that's part of it too. And there is more. That being said, the association isn't a fair one and anybody who thinks about it for 15 mins should come to the conclusion that, in itself, there's not reason to think anything ill of a person's character just because they are goodlooking.
I tell guys all the time just talk to the woman like an adult, see what she is like. Give her a chance and if you are going to reject her, do it on substantive grounds, not because of a bigotry. Just feel her up...I mean out! Out! Sorry, little slip there! ;-)