Let's not turn this thread into just complaining about the behavior of other fandoms (even if it's complaining about the how other fandoms complain)!
That said, yeah, if Ayumi gets in, then I hope people wouldn't be awful to non-fans about it. I'm sure people generally would be salty - she's a first-party that I'm sure would be dismissed as a promotional shill pick - but there's a difference between acknowledging her history and the merits of her inclusion and, like, just dunking on people who didn't get what they wanted...
I also don't really think there's much benefit in trying to convince people any character is a 100%-basically-confirmed lock, as all that'll really do is make people who don't want that character preemptively salty and increase the salt output from people who do want that character if they don't get in...
Personally, at this point, I think there's a giant pool of "logical" picks and it's really just a matter of what Nintendo is prioritizing, so I'm not too hopeful or pessimistic about any individual character...