Those characters don't have those things because you are right, they are too powerful. But this is the Final Smash we're talking about... Below 100% there is nothing final about Cook Kirby; 34% damage and decent (but not KO-worthy) knockback are not deserving of the title Final Smash.
In fact, it's quite the opposite; a little free damage, maybe some combo potential since you've got them in the air, but by no means a kill move like most of the others' Final Smashes. I would support the decision to tone down an extremely powerful ability like Mike or Crash into something still somewhat Final but not godlike. Nothing wrong with giving Kirby a Mike that shoots out sound waves in one direction, perhaps like Mario's only shorter duration and a little more kill-worthy. Maybe he uses all three Mikes at once, in one direction, then the opposite, then an area around him, the first two cause simple damage and knockback and the last one can (but doesn't always) kill.
Plus you cannot deny everyone would love to see Kirby's final Mike pose. Rawk Kirby FTW! =)