I honestly believe mii gunner is top tier. Mii gunner in my opinion is top tier solely due to her MU against some top tiers.
Sheik can often camp with needles, but with mii's forward air, being able to camp, and flame pillar stopping sheiks approaches in her tracks, and the biggest thing stopping sheik is if you space/time bomb drop correctly when sheik tries to grab the ledge she just explodes. This makes the MU about even
Luigi wants to get in with his fireball so he can grab, but you can absorb fireballs, and keep him out of grab range. Also due to Luigi's low traction the tip of mii's side smash is generally safe on shield. overall this MU is about even.
Rosalina is dominated by mii gunner. Luma is smacked away by mii's f-air, shorthop f-air hits over luma, whenever rosy tries to up-air you just use up-b (the one with the projectile), if she tries to absorb f-air just use it again and it will hit her during endlag, and she is the right weight to be juggled by f-air Mii wins this MU
Flame pillar to punish spin dashes... that is all that needs to be said. Even MU
Am I wrong, almost certainly, but this is just my opinion