Hitstun is based on Knockback, which in turn is influenced by a character's weight. Falling speed does not affect the hitstun a character receives.
It seems like floaties escape hitstun faster simply because they fly back farther upon being struck at vertical angles. Falling speed can conflict with Knockback somewhat and give faster falling chars more vertical resistance, but their hitstun is kept intact. By the time you'd reach the floaties that have flew back a fair distance, they are likely to have escaped hitstun. On the other hand, since you don't have to chase fast fallers as far up, they are essentially easier to combo. Both MK and GnW experience the same hitstun for example; they are both the same weight, despite MK being a faster faller.