I thought Mario Kart Wii had some cheap AI. The AI in the original Super Mario Kart is ridiculous. There is basically one rival that is guaranteed to come in at least second place every race, and the other six karts are just there to cockblock you so the rival can win. They do nothing but drive slowly right in front of you, with no regards for their own performance.
I'm currently trying to get the gold on 100cc star cup.
It looks like the rival is determined at the start of each race. In one game, Yoshi was clearly my rival, being on my ass the entire time I was in first. I managed to get a red shell, so I dropped to second place on the final lap and blasted him, dropping him to fourth while I took first, essentially taking him out of the running. The next race, Yoshi was just a drone, while Mario took his place riding my ass the entire race.