You know what makes me scared about this Mii costume? The possibility that Sakurai just doesn't see King K. Rool as a Nintendo all-star worthy of fighting alongside Bowser, Ganondorf or King Dedede. Because besides possibly Inkling (who I think is not going to be in due to being a potential technical nightmare moveset-wise) and obviously Link, all other Mii outfit DLC costumes had been characters who honestly are disposable for SSB4 with their exclusion not being too much of a big deal overall (more Mega Man and Animal Crossing characters, third-parties definitely not at the same level as those who actually are playable, and "deconfirmed" characters). And I'd feel insulted if Sakurai thought that K. Rool is disposable enough to be put at the same level as these before the ballot officially closes, and that it would be enough from Sakurai's perspective to do justice to him.
I mean, even though I'd love to see Rayman playable, I honestly thinks he's very disposable for Smash standards and it's not going to be a big deal for me if Sakurai judges him unworthy of being a fighter. As such I wouldn't mind a Mii fighter costume of him. I would be okay with Wonder Red being a Mii Brawler skin as well; The Wonderful 101 admittedly wasn't that much of a influent title in the long run. King K. Rool, on the other hand, is the kind of character I'd expect to see in Smash for his star power alone, and that's why I'm going to be mad if this costume is going to be enough to represent him.
I don't think K. Rool needs to be playable per se, I just don't think he deserves to be acknowledged as a disposable character. If there are other reasons why K. Rool isn't playable, like Sakurai wanting to do other characters instead, or not liking the idea of King K. Rool using the blunderbuss or helicopter pack or something (which could be possible considering he did separate Mario and Dr. Mario because he didn't like the idea of both having the exact same moveset), or having technical difficulties preventing him to make it in, I'm honestly going to be fine with it. But King K. Rool being downright acknowledged as a lower-tier Nintendo character not worthy of being playable? Sorry, no.
Besides, I'm not going to buy it, but that's because I never buy Mii costumes anyway.