Probably just a lot of "Man, look at the games we've made" and celebrating Persona in a general sense, unfortunately...
Joker has already had a relatively-positive reception; I doubt a FeMC would change it that much. Corrin was disliked for being a promotional pick, sure, but also for being a character from a franchise already seen as having too many characters - in addition to being the protagonist of an unreleased and soon-to-be contentious game. Persona 5 is already popular, a FeMC is a commonly-requested addition, and, frankly, many Smash fans will be happy to have another waifu in the game, even if they don't care about Persona.
I'm sure some people will complain, but with a fanbase the size of Smash Brothers, you literally cannot do anything without upsetting someone....
And, as I've said before, Puzzles and Dragons got full P5 songs. ATLUS isn't stingy with their property; they don't like people illegally* uploading their OSTs to YouTube where they don't profit of it. Dragon Quest, meanwhile, cannot even get non-midi soundtracks between all regions as the composer is on the record as personally being troublesome.
*Japanese Fair Use Law is very different then American Fair Use law, so it's much harder to argue it is legal