It should be clarified the hat actually takes up the head slot of magic items while being worn, otherwise Kirby can wear infinite hats and gain nigh-infinite HD.
Also, Kirby only ever did copy a single ability of an enemy he swallowed, and if we go by Super Smash Bros., then the swallowed creature wasn't even killed by that (the creatures in the Kirby games are more like minions than boss monsters or PCs as in D&D).
Inhale and Maw also don't have a designation of (Ex), (Su) or (Sp). I'd presume both to be (Su).
I'm also surprised that he isn't put down as an Aberration.