Mii costumes have no impact on the DLC.
3DS/Wii U had Link, Captain Falcon, Fox, & Samus and so far we've seen Chrom & Inkling in Ultimate.
You're foolish if you think Mii are murderers.
The thing is that we've never seen a case of a Mii costume being added BEFORE the same character is added to that same game.
Characters like Link and Samus are NIntendo icons so of course some of those get outfits despite being playable, not like there is reason to doubt them being playable.
Chrom's costume was in the last game and Ultimate is literally built off of Smash Wii U, so they're keeping it because it was already made like the other old ones. No reason to take it out just because he was added as a playable character when they already finished making it.
The detail people keep forgetting is that Mii Fighters were introduced SPECIFICALLY for the purpose of "letting you feel like you can play as characters who don't make it in" or "letting you play as characters who are impossible to get in".
Not to mention that in the cases of some more popular characters like Geno (for Wii U/3DS DLC) and now Rex with Ultimate, he specifically makes a point to emphasise that he "hopes you can feel like you're playing as him".
Nothing is a guarantee of course and things change, but for the sake of not getting salty, I think it's better to just take the Mii costumes for what they are and embrace them. I've already made my Nia Mii, y'all can make your Rex or Saki or whoever else Miis.