I do think it's a little soon to be talking about Corrin's legacy - they're like Ike or Roy where there really hasn't been much opportunity to let nostalgia kick in yet. I assume Fates mixed reception is going to mean that, ultimately, they're going to be one of the "also ran" lords. But at the same time, they do still get merchandise, they are in Warriors and Heroes, so who knows.
Byleth is in the same boat, just that Three Houses already has a much bigger impact and a much bigger fanbase than Fates. We'll see how much of an impact they have a few years from now.
I think every game has its own "flavor of the month" characters, though.
64: Captain Falcon, Jigglypuff, Ness
Melee: Sheik, Young Link, Pichu, Roy
Brawl: Lucas, Ike
Smash 4: Corrin, Palutena, Dark Pit, Bayonetta, Miis
Ultimate: Incineroar, Joker, Byleth(?)
Smash 4 is probably on the heavy end of the spectrum, and it's sandwiched between Brawl and Ultimate, which both prioritized "fan favorite" types over all else.
If we're talking fully unique characters, I think Sheik, Palutena, and Corrin are the most "wait, why?" characters in Smash at this point. Sheik and Palutena in particular are supporting characters, Sheik having never reappeared and Palutena only REALLY appearing in one game in a fleshed out role. But they're also some of the only female characters in the game, so I can't complain about squeezing either of them in.