I think the main difference is just timing. Yoshi, Wario, and DK got their own series relatively early in the character's lifespans. Super Mario World 2 was one of Yoshi's earliest appearances, same thing with Wario Land for Wario. The DKC series is a slightly different case, while it came more than a decade after the original DK arcade, DK and Mario were more equals due to debuting together, DK hadn't really had much involvement in Mario's main platformer series, and DK himself as a character had already been appearing without Mario in stuff like Donkey Kong 3 arcade and stuff.
When Luigi's Mansion came out, Luigi had already been a Mario character for dozens of Mario games and more than a decade, and the main deuteragonist no less.
Actually, that isn't true for Wario. He got his own character only a bit after WarioWare debuted, while Wario Land was going strong for years. He only had a trophy in Melee under the Mushroom Symbol and nothing more. We don't know if he could've gotten a Wario Land symbol either. He just debuted in Smash as playable when his new series was fresh. Yoshi is about the same, having two games under his belt instead.
As I said earlier, Luigi could've been Luigi's Mansion if the stars aligned for it. They quite clearly did not.
Also, doesn't Mario Kart have its own section(not symbol) pretty much due to music only? And I forget which game started that. Mario is such a massive series it has more music than generally anything else, so the split is probably more for data reasons than even Mario Kart being a huge spin-off on its own.
Technically Big Boo is still used in Super Mario 64 DS, King Boo actually gets his own fight since you have to enter a Luigi painting and platform/find your way to where he and King Boo are, although I guess the fight is similar to Big Boo but there was a unique spin on it with the mirror iirc.
Ah. Thanks for correcting that. But yeah, unless the remake is supposed to be the definitive edition, I don't think it's nearly that meaningful. I'd like to know the SPP role further, but it's a small role in Sunshine too.
Besides, Trophies and Playable Characters are different. King Boo being based upon Luigi's Mansion? There's nothing that really should make him fall under Super Mario at that point. He's not about that. Same with Gooigi. E. Gadd, as I noted earlier, is important to LM and SM, so either symbol would work in a respective manner. But it also depends upon what his abilities are. What has more focus? The content he comes with obviously would depend upon this, not the other way around, so.
Incidentally, isn't Luigi's Mansion the first actual true Luigi course? Ironic.