So there used to be this Tumblr blog called "Smashing Is Magic" (which is sadly no longer active) where the Mane Six and Spike got together to played the Wii U Smash Bros. At one point I asked them about what Mii's they'd use and they showed me the results along with their moves and their reasons for choosing said moves!
Twilight Sparkle: Twilight chooses the Gunner for because out of the Mii’s, it’s closest to use of Magic. The Side Special and Down Special are also a bit close to Magic because it’s like throwing a short fire blast, and the shield acts just like Ness’ down special as it absorbs energy projectiles and heals. The recovery was a bit harder to choose as neither fell very magic-esque. So she just went with the one with the rocket boost down and a big fist in the air. Her Mii has the Magic hat and the Mage clothes as an option further supporting why the Gunners work best for her.