I don't know... if Kirby's Hammer did 40% with a sweet-spot it might make it almost useable; and, I know the overhead swing is Kirby's basic hammer swing in Superstar. It's what I expected from a basic swing in SSBM too.
The best way of fixing this is to change Kirby into Hammer Kirby with Copy.
Hammer Kirby
Neutral B: "Hammer Nail"
A basic, overhead hammer strike (15% attack).
Smash B: "Hammer Swing"
Kirby spins on the ground as seen in Super Star (Ice Climbers' Squall Hammer). Use Hammer Swing in the air to spin in a circle (as seen in Melee, but gains horizontal distance by tapping B).
Down B: "Gigantic Hammer"
Looks like Hammer Nail, but hold B and the hammer quadruples in size! Strikes when you release B, as seen in Squeak Squad (18% version of Hammer Nail)
Up B: "Hammer Flip"
When used on the ground, Kirby charges up his hammer with flames and then strikes with a powerful swing (26%). Same thing as when used as an aerial attack, but he moves forward as he swings.
Taunt + B: "Hammer Throw"
Kirby throws the Hammer forward like an olympic star. Acts like a Final Cutter projectile. This discards the ability.
Quick Notes:
- If Hammer Nail had decent range and useable speed, it could give Kirby his needed KO attack.
- Gigantic Hammer seems like it would work similar to Link's bow, but instead of shooting an arrow, he just has a big hitbox to strike with in front of him.
- Hammer Flip would be similar to a Falcon Punch, I guess.
- Hammer Throw is inspired by Kirby Super Star where he throws his hammer at his opponent if you discard your Hat at the same time you attack.