Is MarioKartOutfit "C" real?
In short, no. Multitudes of people have modded their ISOs for use when running an emulator such as Dolphin to create a custom racer or to change the appearance of one of the already existing racers.
From Super Mario Wiki's Mii page:
Originally, a Mii Outfit C was going to be added, but was cut from the final version of the game.