First of all: Even if K. Rool tops off the ballot votes like you seem to think, that doesn't mean that the ballot winner is going to actually get put into the game.
Second: Nobody really cares about him being fat and ugly - his visual design doesn't matter anyway. The point most people have made against K. Rool is that he simply doesn't have the relevance that Dixie has. Beyond spin-offs like that Mario Baseball one (which was released 7 years ago and the last appearance of K. Rool iirc), he hasn't appeared in any actual games since Donkey Kong 64 (and the puzzle and racing games are spin-offs, mind you).
Dixie Kong, on the other hand, has appeared as a playable character in the most recent installment of the Donkey Kong Country series (Tropical Freeze), and has had as many cameos in spin-offs as K. Rool, if not more.
To top it off, Dixie was planned for Brawl, but the partner mechanic with Diddy was ditched for an unknown reason. This means that Dixie clearly is on Sakurai's radar as well.
Now here's a funny thing: I could also apply the same logic you are doing with K. Rool's Mii costume to Dixie: Look at her trophy in the new Smash game.
This is, in fact, an entirely new model in a pose that has
never been used before. Google it yourself, if you don't believe me.
Now, I was being mocked about this before for bringing this up as a hint that Dixie might've been at least planned to be playable, but here's the question I'll pose to you, UglyRareCharacter:
Would Sakurai and his team have gone through the process of making an entirely new model for Dixie Kong if they could've just nicked the Tropical Freeze model, and all that
just for a trophy?
Think about it.