Well, while tumble is defined by bouncing, rolling and zig-zagging on the ground, aerial creatures should use it analogously - e.g. by aerobatic stunts, like barrel rolls, loops, sideway slipping etc.
Although I would limit it by the manoueverability of the creature with poor flyers being able to use only a very low maximum number of ranks, increasing to any amount (or maybe a bonus ) with (perfect) flight. After all flight does come in differing varieties of skillfulness, unlike walking.
If we are talking mount and rider combinations, I would be very leery though - either the mount is in charge and "aerobatics" in, which would mean its rider will likely have a very hard time hitting anything as he clings on, or the rider is in charge which would mean his directing efforts would hamper or cancel out any "innate" aerobatics/tumbling skills on part of the mount.