I think Corrin pretty much completely ruined Fire Emblem in the eyes of Smash fans.
Like Fire Emblem really shouldn't of gotten another character in Smash 4 after it already got Robin (+Lucina as a bonus) in the base game, and also especially how they just added back Roy in the same DLC cycle. Smash 4 just pumped in way too many Fire Emblem characters way too fast and now it's permanently damaged the series' reputation amongst Nintendo/Smash loyalists.
Really they should of just never bothered with Corrin and also should of just held back Roy for Ultimate (where everyone was here anyways), just use Elma and Wolf instead for a new post-Smash 4 Nintendo newcomer and a popular returning veteran respectively. Had they done all of that, I think people would of been far more accepting of Byleth in Ultimate (though I imagine some people would be raising their eyebrows at FE having seven characters, but again, that's mostly because of "Everyone is Here" + Chrom being a popular bonus that was easy to add), because by that point it'd had been several long years since Fire Emblem got an all new fighter in Smash.
I'd have to agree, though I think that Corrin would have been accepted a bit more had Roy been saved for Ultimate and the series as a whole would be better received if Sakurai hadn't made the frankly terrible decision to have half of its reps have mostly the same moveset..like, seriously.. for original movesets with have Marth, Ike, Robin, Corrin, & Byleth, but then we have Roy, Lucina, and Chrom who all share movesets primarily with Marth..had Sakurai chosen to change up Lucina more in Ultimate and made Chrom an Echo of Ike instead of an Echo of a Marth Clone, it may have fixed it a bit more as well.
But yes, its largely 'EVERYONE IS HERE' that, ironically, caused a lot of the issues here. Fire Emblem is in a bit of a darned if you do, darned if you don't situation..Like everyone wants EVERYONE to return, but that also means that Fire Emblem, as a series that has been in since Melee with a revolving cast and multiple Echos/Clones, would have a disproportionately large cast where most of them would have ended up cut otherwise.
I think that one of the ways to 'fix' this issue is to have all Clones/Echoes share a slot, much like we can do with Echos already, including characters like Roy & Chrom under Marth as well, so that when you choose Marth he has 4 overall options for moveset signified by Marth, Roy, Lucina, and Chrom.
This would make the Fire Emblem Reps technically drop from 8 (Marth, Lucina, Roy, Chrom, Ike, Robin, Corrin, Byleth) to 5 (Marth, Ike, Robin, Corrin, Byleth), which might be more palatable for people.
I think that the series stigma will stick for a while, and I blame Sakurai continually making Marth clones..like I recall people WANTING Roy to return before Lucina was announced as a new Marth clone.
In short, I think people's seeming hatred of clones prior to the term 'Echo Fighter' mixed with how many of them Fire Emblem had and thus people begun associating their issue to be with Fire Emblem, as a series, rather than the actual issue they were having.
Some would, and you can’t please anyone. But a lot of people who dislike JRPG guys do so largely because the genre is so highly represented in Smash, while we don’t have any characters from FPS’s, Rythm Games, Puzzle Games, phone games, etc.
It’s not just FE though, I’m a supporter of cutting characters from several franchises if they’re not going to keep the everyone is here theme going for the next game anyway.
To be fair, FPS games aren't really popular in Japan, Puzzle Games are a bit difficult to represent in general (though we do have Dr. Mario as a Puzzle Game Rep), Rhythm Games could similarly be difficult to accurately depict in a fast paced game like Smash, and you may be the first person I've ever seen want mobile gaming reps.
We are also missing visual novel reps, which could be repped by Phoenix Wright, Ayumi Tachibana, or, my personal choice, Saber from Fate/stay night.