He is one-half of the middleweight class in the game. He is easy to handle, has admirable acceleration and speed, and is one of the most balanced racers in terms of stats.
The leading man in nearly all of Nintendo's most iconic games within the Super Mario canon is the best playable character in the N64 iteration of the Mario Kart franchise-for beginners, that is. Without Mario, there would obviously be no Mario Kart.