Guys, please do not panic.
Mii Costumes do not mean anything. Miis are not meant to replace characters.
Look at all the gear that's in the game from the get-go:
Mario, Luigi, Wario, Captain Falcon, Samus, Meta Knight, Sheik, headpieces, and more!
Even a full Link costume is there for DLC. It's just dress up.
I agree that it's kinda weird for Nintendo to announce a King K Rool costume, but think about it this way:
Nintendo obviously knows about the K.Rool hype. What if the costume really is just a tease to sweeten the wait? Sakurai stated that characters take a lot of time to make, especially when it comes to newcomers.
Now look at the costume, the scale details and the golden belly textures look nice, too nice for a simple mii costume tbh. I'd rather believe that this costume is just a taste of what's to come, Nintendo isn't stupid after all.
They know about the hype and they know to better not anger the community. We pay their bills, after all.