i don't see any POC actually complaining though? like, representation is one thing but it's another to outright call something racist as if it's a problem, it's not

Mario Kart's had the same characters for years as a reflection of the regular Mario games. if you're going to say something about representation then demand a more diverse cast in the official Mario games so there are more options available in the spin-off games like Mario Kart and Mario Party.
complaining about something as generally trivial as MK doesn't help get the point across. the game isn't necessarily meant to represent any one thing, it's just a game that let's you play as any Mario character you want.