Let me put it like this: for those unaware, you don't go into a League match just by picking your character. You choose (or get assigned) a role in the field, then declare which champion you want, then there's the banning phase (aka you take out who you'd rather not play against) and THEN you pick your character. It's simpler than this in Blind Pick matches, but you get the idea.
So, that banning phase part? Yasuo is consistently in the top 5 most banned among both casual and competitive matches
since July 2016. And no, it surprisingly isn't
all related to just being good in higher elo. It fluctuates a lot (in part because of the meta), but that's how bad it is for him. There's at best two or three other champions out of the 150+ that can boast that same claim.
Tldr: if a LoL character got in, I can see a part of the Smash fandom being salty.
But if Yasuo gets in? I can see both a part of the Smash AND LoL fandoms being salty.
This Reddit post compiled some pretty decent info, as well as a link to a YT video with the graphs since 2016, in case anyone's interested.