While I don’t think any of the Mii Fighters are top tier characters, I would say Mii Gunner is certainly the best one of the bunch.
Mainly because the Mii Gunner has the best moves of many standard fighters. You have a very similar laser orb like Samus Aran shoots. And a fountain of flames that is pretty close to Ness’ PK fire.
And a move like Fox’s reflection barrier.
Unlike the other two Mii characters, Mii Gunner has a great recover. Mii Gunner won’t fall to his/her doom like Mii Brawler or Mii Swordfighter.
Their final smash is really good, very Samus-inspired. A trio of large laser blasts. They don’t take your opponents as far off the edge as I’d like during stock matches though.
When Kirby takes Mii Gunner’s power, he gets the laser ball power. One of the best ones to take. Though it should have more power to it.
While I liked Mii Gunner I didn’t quite love them. It is fun to dress them up in costumes, but they are lacking a bit in the smash attacks and agility. So I did fair with the fighter, but far below my best fighters such as Palutena and Bowser.