How do Mii Costumes work in Mario Kart Tour?

gamepur.com:

Miis live on in Mario Kart Tour where your personalized character joins the roster of growing Mario characters in different outfits. These choices should be near the top of your list if you are a player who prefers to inject a little bit of yourself into the game. That being said, the stats of your Mii will depend on the costume they are wearing. Here is how Mii Costumes work in Mario Kart Tour.

Unlike usual Mario Kart Tour characters, your Mii’s favored courses and skills can be changed depending on the costume they are wearing. Right off the bat you are given the Red Mii Racing Suit, but you can purchase, unlock, and equip costumes based on Mario, Donkey Kong, and other characters and color schemes. More suits can be unlocked through future tours or bought in the in-game shop.

To keep things simple, Mii Costumes in Mario Kart Tour are essentially different characters, you can just swap your Mii between them to find the set that you like best. Your Mii is a customizable face that you can put on the body of the character in that outfit. You can change the outfit they wear at anytime in the Mii portion of the menu.