While there are quite a few Mii costumes in the base game of Smash, there’s only been one bonus Mii costume made available through the Fighter Pass. Smash 4 included tons of paid Mii costumes as DLC, and it seems to be that Ultimate may be doing the same.
Listed on Nintendo’s support website is a list of available DLC both current and upcoming. This includes the fighter pass, individual characters and the surprise addition of Mii costumes for $.75. It’s unknown what all this includes as Nintendo themselves have said nothing about Mii costume DLC previously, but we may be getting more in the future as Smash continues. The date for release is just listed as TBD in 2019, which means the next time Nintendo updates Smash through a direct or other means might enlighten us further on what we can expect.
Those interested can check out the official page right here.