From Comic Book Resources:
In the Japanese version of Mario Kart 64, the Flower Cup's Mario Raceway bears several advertising posters for a company called Marioro. As its layout and poorly constructed portmanteau name would suggest, it's none other than a Mario themed version of cigarette company Marlboro. This and other ads were placed throughout the game's tracks for a sense of immersion, making them more like real-life racing events. Marioro's ad doesn't show any of the company's potential products, but it likely sells smoking products. For this reason, in particular, the ad was changed entirely in the Western release to "Mario Star," as a cigarette ad wouldn't exactly be appropriate in an otherwise colorful kids game.
Several other ads resembling real-life companies exist in the game, with some even retaining their original names. These include Luigip, Yoshi 1 and Koopa Air, which are based on racing sponsors Agip, Formula 1 and Goodyear, respectively. The first two had their names changed in the Western version of the game, with the also altered color schemes diminishing the resemblance to the logos for the companies that they were based on.
While the cigarettes are probably a bigger deal, given that Mario is the main protagonist, I find Yoshi, a dinosaur, shilling for fossil fuels to be even more disturbing.
Super Mario Has His Own Brand of Cigarettes in Japan [Timothy Donohoo / Comic Book Resources]
Mario Kart 64 once had parodied tobacco ads [Super Mario 128 Central]
The Japanese version of Mario Kart 64 has parody billboards, including a knock off Marlboro cigarette logo. The billboards were switched to generic Nintendo ones in the US copy presumably for legal reasons. [Reddit]
Image via YouTube