Well, playing devil's advocate here, people generally want characters from games they like already.
It's hard to swallow when a character gets in not because people like the game, but because it was in the right place and time to serve as a convenient advertisement.
In general, people like when you use a fighting game cameo to appeal to fans of the game, rather than using a fighting game cameo to appeal a game to the fans. It's a mix of disdain over the "corporate" side of game development, exasperation at over-saturation of something they personally aren't interested in, and disappointment that the character wasn't someone they were personally interested in.
I will say then, it's very fortunate for Smash that Roy has Melee Nostalgia and Byleth's game was critically acclaimed. Corrin then at least has their water dragon powers for the Smash players that like obvious character uniqueness.
Personally, I would've gone with Dimitri first or Edelgard second before Byleth third, but I imagine non-FE3H fans would still be pretty mad about that.