Let's have some fun and blow the whole "Herp, if you think the costume makes K. Rool likely, you must also think [insert character here] is a shoo-in!" song and dance out of the water.
Warning, pretty massive image ahead:
This is a comparison of every first party DLC Mii costume with the base from which they were modeled, mostly from Smash with one or two from outside. I do want to emphasize that this is intended strictly to demonstrate the -bases-, as clearly not every costume's proportions and textures are 1:1 translations from the source model. Stuff like Black Knight clearly got upgraded and sized up, while Ashley and Takamaru got great new textures too (that I'm curious to see compared to the latter's AT model in Ultimate once higher-quality footage comes out).
This also does omit third party characters, whose situations vary a good amount. The Mega Man character costumes were largely pulled from the characters who appear in Mega Man's Final Smash, while Geno is blatantly a new model, and a handful of the others may or may not have been costumes the various third party costumes put together and contributed so as to get more of their work in the game.
Point being, K. Rool's costume is a glaring exception among the other first party costumes in the game, even though his Paon model is sitting right there. It far from guarantees him a spot, but it's a weird circumstance we've yet to get a good explanation for. People who diminish this are largely either unfamiliar with the character's modeling history or are just oblivious/going "la la la" because downplaying Rool's chances makes them feel good.