Edit: I did my little calc when I didn't totally understand the j/cc method. My retry:
Looks like fragmentation, so 8 j/cc
373,248,000,000 joules. Using the same converter I actually got 89.21 tons, a much higher result. This would make it a City Block level feat.
But if it exploded after this, into much smaller pieces (it would've helped if you gave a YouTube link or something, but nonetheless,) it'd be violent fragmentation, so 69 j/cc. 3,219,264,000,000 joules, or 769.42 tons. Multi-City Block level+.
Finally if it WAS vaporisation (which I HIGHLY doubt), it'd be 25700 j/cc. 1,199,059,200,000,000 joules, or 286582.03 tons, or 28658.203 kilotons, 2865.8203 megatons, or finally 286.58203 gigatons. In that case, it's Large Island level. The hiugh result and because you didn't provide a YouTube link leads me to believe this is definitely not the case, but whatever.
I apologize for not seeing the original. I'm no calc group member, but sure.
58.2857 m is 5828.57 cm
For fragmentation, 378.6 tons of tnt, instead of City Block it's MCB.
For violent fragmentation, 3265.45 tons of tnt, instead of MCB+ it's Small Town level.
For vaporization, 1216263.01 tons of tnt, which is a LOT higher than ever anticipated. 1.21626301 megatons of tnt, or Small City level. This and the Mii Fighter page can be used as reasoning for Low 7-B Composite Mii.
There are absolutely no reasons for the Mii Fighters to NOT be included in Composite Mii. It's not remotely comparable to stuff like Ice Climbers or Captain Falcon, because there is no "canon" Miis. Their entire point of existence is to be a stock playable avatar in many games, Smash being just one of them.