Not everything can be killed by jumping on it. It seems more reasonable to assume the things that you can kill by jumping on them are just weak to that, rather than the Mario Bros. having some sort of intrinsic property that makes them OHKO things they jump on.
Box taunt has a hitbox.
Maybe give Bowser his old ledge jump back? I kinda liked how you could get onto the stage quickly. If not this, you could just speed up his regular ledge get up.