Skull bash's only offensive use is as an aerial 'dash attack', in that it can hit if a foe commits to an attack that cannot hit and you're able to punish. The best example I have is if a ZSS gets stuck charging a paralyzer that you're already above, or the like-- landing would be unsafe, double jumping would lose the window of opportunity, while skull bash can get a poke to her head while skimming over the paralyzer shot. If you ever screw up spacing in the air and need a gap-close, it functions as a dash attack in that regard: very unsafe and laggy, but if your enemy is guaranteed to be hit, it's potentially your fastest approach. That said, it's still really just meant as a movement utility-- I've done some testing when looking at Pikachu's recovery options by coincidence, but it only ever seems at all to have a purpose in gliding over projectile edge-guards that are done from too close or the like, which /happens/ to hit the foe. Though, it's sometimes a mite safer than quick attack, given that it at least doesn't put you into special fall.