Of course it's not a real game.
We doubt there are enough functioning SNES controllers left in the nation to get 101 players into a game of Mario Kart.
No, this is a superb piece of YouTube fiction, created by Hat-Loving Gamer, imagining the sort of chaos that would unfold if you were able to give almost every Nintendo character a spot on the grid of the original Kart game.
It's almost as bad as the M25 on a Friday evening. Almost.