Mii (Outfit B) is a character that let's you use your Mii. It's gender can be Male or Female, and it's weight class is determined by it's height and weight in the Mii Channel.
The look of Mii (Outfit B) is different from Mii (Outfit A). Depending on the gender, your Mii will wear a plumber (Mario) suit if it is a boy, or a dress (Peach) if it is a girl, instead of a tight-looking driving suit. If your gender is male you wear mario or luigi overalls acorrding to your preffered color.
Unlocking Mii (Outfit B) can be very difficult to unlock, because you must unlock all 32 Fast Staff Ghosts
Same as Mii (Outfit A): For Light Miis: Top Speed +3, Weight +3, Drift +3, Mini-Turbo +3. For Medium Miis: Top Speed +3, Weight +3, Off-road +3, Mini-Turbo +3. For Heavy Miis: Top Speed +3, Acceleration +3, Handling +3, Drift +3.