The only times we've ever had an actual theme is with Hero (Dragon Quest) and Terry (Fighting games), and to a lesser extent Joker (Sega). We're more than likely going to get character later on and find that an older Mii Costume would've fit with them better.
While those Themes are more Pronounced, I do there there is possibility that all of the Costumes so far were Deliberately Placed in their Specific Pack because it fit with what the Character is about, though it really depends because I also do think it's likely some spots were a Dumping ground
Banjo was a Requested Character, Just like Sans, Zero, and to a lesser extent Proto Man and Goemon (who's the Japanesse Mascot of Konami Apprantly).
Byleth is known by their 3 Weapons, their tournament is based around it, so their pack having all Mii Types is Important.
Min-Min is a New Nintendo I.P, and also a 3d Fighting game. Splatoon Costumes and Heihachi just stress this. Also Ninjara as an ARMS Character
Steve had Minecraft, and Gil's Game is heavy on Treasure Collecting, which is big part of Minecraft, finding material like Gold and Diamond. Bomberman is the Opposite of Creation, being destruction. Travis comes from Indie game, which is what Steve was
But Like I said some spots are likely just Dumping Grounds, like Vault boy going with Min-Min, and Team Rocket with Banjo. There also might be looking into it too much, but I do think they put more effort to placing the Miis than we give them credit for