I wanna see a Smash character who uses Pichu's "hurts themselves" gimmick, but like actually taken seriously. What Nintendo character would fit though?
While Pichu was pretty much trash and a joke, the idea can work. There was a fighting game released about 2-3 years called Under Night: In-Birth which featured a character called Carmine. His playstyle was that his special moves used up his own health. In return, he was very strong.
Too strong, indeed. In the first version of the game Carmine was borderline broken. It didn't matter that he hurt himself with his specials, he had such amazing stage control, damage, and combos that he was pretty much the best character in the game till the devs clocked him with the nerfbat in the later editions. It didn't help that he had a passive health regen (it was suuuuuuper slow, but still enough) AND could restore health with a throw and a super (they got rid of the passive regen for later versions it seems). Even after the nerf he's still a pretty damn good character.
A character like that in Smash, especially P:M, would be interesting. The ultimate risk-reward character: you're overwhelmingly strong, but you can't waste any specials and must end it quickly because if you don't you'll put yourself in kill percent for the opponent. On the other hand, if your opponent smacks you around to kill percent, you might as well go nuts since the difference between, say, 140% and 240% doesn't really matter, you're gonna die quickly at both. On second thought, this might need some work to actually mesh well with a percent platform fighter.
I actually have a character like that in my platform fighter, but given that it exists only on paper, that's not really much to brag about.