The first time I remember seeing any kind of cheerleading was Bring it On in 2000 and they had crops and skirts. So yes, I would say it's traditional for at least the past 14 years. Just like it's traditional in gymnastics to wear a leotard, wearing a speedo in diving, and little dresses in figure skating. For the most part, cheer society has leaned towards crops and skirts. Not sure if you're actually interested in why people are ok with things being a certain way but there's a bunch of studies in psychology that say people go with the norm and obey people in power (officials, doctors, coaches, etc.) so I'd suggest reading those if you really want to know why. Gyms can choose to put their athletes in sweatpants and turtlenecks but tasteful crops and skirts are alright too. If you're talking about tops that barely cover underboob and skirts that are shorter than spanks, which is what I think you're getting at, then no that is traditionally not ok.