Yeah Goombas don't have hands, so what? It's about time they get the upgrade from obstacle to driver. How will it drive? Who knows? You're already playing a game where turtles, mushrooms, and squid people are kart racing, so is that really the question you should be asking?
Goombas are the first enemy encountered in the very first Super Mario Bros., before Koopas or ParaKoopas and long before King Koopa Bowser himself. As such, Goombas have become some of the most well-known and iconic enemies in the entire Mario series, appearing in almost every title.
Maybe Goombas don't have hands, but that just adds humor to them driving a kart.