I have went to many websites looking for an answer and I got many responses. For example, the Wikipedia said that there is a way to unlock mii outfit C while most of the other websites concluded that it was either a glitch or not real at all. Does anyone know if mii outfit C is real? If it is, I'd appreciate if you could tell me how to unlock it. Thank you for reading!
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.
Originally, a Mii Outfit C was going to be added, but was cut from the final version of the game.