The near universally agreed upon competitive tier list for Smash 4 had Ganondorf as the third worst character in the game, and Zelda as second worst character in the game. Granted, they may have scored a bit higher if the Miis were accounted for, but not by much of an increase if any at all. Short answer, it's near impossible for them to do worse than their Smash 4 performance when they both were bottom three, but then again, Smash Brawl managed to pull that off, so it's not impossible.
Plus, the game has only been out for 33 days, and the competitive community still needs a lot of time to establish who truly is the best fighter as they did in every Smash game. In Smash Melee,
Fox wasn't revealed to be the best fighter until 2006, and
Zelda was actually seen as being one of the top ten extremely early on. With time, it's likely that a fighter we thought was high tier will actually turn out to be crap. It's also likely that the opposite will prove true as well. There's also the possibility of balance patches which can really throw a wrench into the system.