I think its because cheerleaders are very outgoing and loud.
I personally don't think it's bad however, from a girls standpoint: when their boyfriend is on a football team, or basketball or what have you and another girl is "over excited" when that girl boyfriend makes a basket or scores a touch down they get jealous. ~its their job as the girlfriend to be excited and another girl is trying to be better at it by wearing short skirt and jumping around~ Now i know that is not the goal of a cheerleader, it just to encourage their team and try to keep spirits up. Some girls just cant stand it and start to cause drama, then everyone starts to think well yea, why does she have to cheer my man on, he has me. and it turns in to a catty little problem.
I dont mean to offend anyone. I do not think cheerleaders are all bad. I do not think that all of them are "whores and what not" i think that people should know some one individually before making statement because of a stereotype.