While I like playing as Chrom, Lucina, Robin and Byleth, I think the problem of Fire Emblem characters in Smash is that, while a bunch of them have interesting or unique gameplay styles, thematically they look the same, making them artistically derivative. This includes:
-Anime artstyle
-Very young characters
-Being conventionally attractive
-Being conventionally slender
-Holding a sword
-Wearing a cape or a coat
-Typically blue-haired
As a result, the Fire Emblem characters in Smash get a bad rap. And who can blame them? It looks like a starter pack of designing a character in Fire Emblem. And even if you suggest other Fire Emblem characters, they would fall into de trap of being similar to these other characters, going back to square one.
And I like Byleth. I just wish she looked more different.