Honestly, Granblue in Smash, with the current way it is, feels like one of those cases where you're trying to force a square piece into a circular hole of a puzzle.
Corrin, arguably the most controversial Smash inclusion to date, still had more things going for him at least. He was a rep for a well known series, brought nice enough new things to the table, and at least didn't feel completely out of nowhere, since his series isn't nearly as nestled deep down.
Granblue just feels like it takes more work to justify, given its lack of worldwide appeal, been-there-done-that aesthetics, and it's a mobile gacha series sporting a character that most people didn't know or want until some leak that went bust came along.
Basically, Granblue, until it makes more of a name for itself, feels like a choice that would get even more scorn than Corrin did, and for not a great net result.
Even Erdrick at least has the fact that DQ has a notable enough presence world wide. And the infamy of gacha games doesn't help either. It also doesn't help that Granblue is one of those cases where the players have to do more research just to get them. I mean, in the case where it's not that well known or readily available.
Like, why even bother with the controversy at that point?