Swerving potshots from the peanut gallery, here's where that stands:
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This is still 5 of the 7 and the other two are MIA. The reason why Mii Costume Guy is so notable is that Mii costume information is incredibly hard to come by. It's a very niche thing and for someone to not only produce a list but bullseye left field picks like Goemon and Rocket Grunt is extremely impressive. In Smash 4, Protoman came with Megaman X, not Zero, so there was no reason to expect this pairing either unless they genuinely knew something. That's why this one example is do peculiar. It would be assinine to assume it was a lucky guess and being correct for 5 of the 7 while two are merely missing does not imply bad information.
Beyond this, there's other things supporting at least the Mallow hat. Cacomallow is still definitely a thing and it took the modding community a whopping 8 months to even figure out to modify Mii costumes to the degree Cacomallow did, nevermind actually replicating it. Other people have heard about the Mallow hat too including people connected with Papagenos. This said, the Mallow hat isn't an MCG exclusive, in fact, it's the only one he's named that isn't. "Well, what if those other guys are just lying?" Based on who they've come from, I genuinely doubt it. This said, the Mallow hat is just one of many, many loose ends at the moment.