If you asked me whether I wanted Tiki or Corrin as the playable Manakete unit in Smash, I'd go with the former, but that's just me. People don't like Corrin due to the writing in Fates and because their inclusion in Smash 4 DLC ended up being a bit premature but at the end of the day their moveset is still plenty fun so I can't really call them the worst addition to the game.
The actual worst addition to the game is Mario. Dude has some real boring platforming games and I swear he's EVERYWHERE, I'm so sick of not being able to look two inches left or right without seeing that failure of a plumber. Just cut him and make him an alternate costume for Dr. Mario, then everyone is happy.