there should be a category where you start a new save file and get mii outfit b

yes!

It would be nearly identical to the 32 Tracks time trials category: https://www.speedrun.com/mkwextracategories#Time_Trials

It doesn't make much sense to duplicate it.

The only difference between this and 32 tracks TT's is just unlocking all of the tracks. I'm not sure this is worth a category.

Also, there's another way to unlock the Mii. Mii Outfit A requires that you win in the 100cc special cup in grand prix. Maybe we could combine the requirement to beat 100cc special cup and then get a fast staff ghost on all tracks in time trial to get "Unlock both mii outfits."