So there's this Tumblr blog called "Smashing Is Magic" where the Mane Six and Spike get together to play the latest Smash Bros Game. I recently 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):

Gunners:

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.

Rarity:





Rarity's Special Moves are completely different from Twilight's. Although 2 of her moves are similar to two Samus’ and Fox’s. She plays as a gunner in order best to keep enemies at a safe distance. After all, we don’t want her getting her suit all dirty when fighting now, do we?

Spike:




Spike's special moves are a bit of a combination of Twilight's and Rarity’s. He has Rarity’s Mii’s Neutral and Down Special. But also has Twilight's Mii’s Side and Up Special. Not to mention the Dragon Armor fits very well!

Swordsman:

Rainbow Dash:





RD & Fluttershy use Swords because true Swordsman treat Swords as part of our bodies. And Pegasi have Wings that really are parts of their bodies. The Swordfighter also happens to have at least one Weather related move as options. Like the neutral special Tornado move. For RD when she feel's like Spinning like Sonic there is the side special, then there’s the quick slicing Up special, and the down special that has a Rainbow and acts like Mario’s cape!

Fluttershy:




Fluttershy's set is completely different from Rainbow’s set. While Rainbow’s is a little more close and personal, Fluttershy's has more of a "keep away" with two types of Projectiles, a Spinning recovery similar to Link’s, and a Counter.

Fighters:

Pinkie Pie:






OOOOO I threw a ball! Slam in the air! Hyper kicking motion in the air! And while that looks a little painful.. a headbutt ground pound!

Yeah, I chose the craziest looking moves in the Brawler’s repertoire thought it’d fit you. Also how do you like the track suit along with the silly small hat?

Really cool! I bet you did the Track suit because I sometimes exercise at home. And the little hat is just the best! Thanks Twilight!

Applejack:







Earth Ponies naturally get Brawlers since they’re relatively known for great physical strength. All of AJ' moves use her legs! It only fits with somepony like her with the strongest legs to have all leg-based specials here.