One thing that I've noticed from watching dk3's True Arena WR run is that they're able to do insane amounts of damage with the hammer ability. Is there some sort of regional version difference where the bosses have less health in the Japanese version, or is the hammer ability stronger in the Japanese version? Should I buy a Japanese copy of the game?

I can't find any info about regional differences in Kirby: Planet Robobot, so I figured that I might as well ask.

There are no differences in regions