To quote what I said elsewhere...
While alternate costumes are appreciated, they're just that, alternate costumes. They're basically consolation prizes, not unlike Injustice 2's Premier Skins (John Stewart, Black Lightning, Vixen, etc.). They're still nice, but they're far from sufficient.
In Dimitri's case, as much as I like his story arc, he's just another white guy (the last thing Smash needs right now). I mentioned Edelgard & Claude because, in addition to being unique mechanically, they'd help with Smash Ultimate's gender & ethnic diversity problem (respectively).
I know why you named Edelgard and Claude, and I am implying the problem for some people here is not Byleth as a fire emblem character, it's just that he is a white. That's why Dimitri also gets forgotten by the people who complaint about Byleth, despite not having any sense to put one lord and forget about the other two. So at the end Sakurai chose the only character that can represent the whole game in its own (who are a man and a woman!!) and gave them a moveset based on the three lords relics.
What Smash needs are new characters with diverse and creative gameplay mechanics to keep the game fresh.
However, let's push the Byleth is another Marth just because of the hair colour.