Can you fall through a cloud?

No.

because clouds are fluffy

Um, ya i guess clouds are "fluffy" but thats not why we can't stand on them haha. Clouds are just made of water. thats pretty much it. water and air. now if u learn how to stand on air and/or water, then go for it! and that would be awesome! but i personally can't do that =(. so, no. it's not possible for people to stand on clouds.