I see, thx. Well yes the stomp can be used mid-air and yes you are also sort of invulnerable during it unless the enemy itself is not stompable like the flying spiky enemies in level 3 and of course wallspikes will also still hurt you even during the stomp. It's very strong but it is somewhat balanced by the fact that you cannot really stomp enemies above you so at the very least you have to get above them first or next to them if there's a floor.