From the Super Mario Wiki
Mask is an offensive item found in Mario Kart Arcade GP 2. The face is very akin to the Koopa Clown Car face. Like many other items in this game, it locks onto an opponent once thrown after it has aimed. When hit, the opponent abruptly halts, and the opponent's face is then covered by the mask. The mask varies in aesthetics, but all serve the same function. Like most other items in the game, it eventually fades away after a set amount of time.