In the beginning of the movie (during Torrence's dream) at the end of the cheer, where Torrence's clothes fall off, you can see that her underwear are showing beneath the pom-pom.
Even if it was her underwear, who cares? Did you honestly think she would do the shot totally naked? I think she did a pretty good job of hiding herself with only pom-poms.
When the crowd is screaming they are clearly just talking about her upper body. I don't know about you, but if I was just in underwear I would cover myself up to. So they could have just wanted her upper body to be seen and the underwear thing was done on purpose.