Solid idea, I really think you did a great job on the graphics.
I wish the rules were explained a bit more. I popped into the first race and was pressing buttons to see what everything did. I don't think it hurts to leave "Press X to accelerate!\nPress Z to boost!" in something like a jam game, especially.
I was hoping for a little more feedback. I saw there was a "Third place" time in the bottom right, but that didn't seem to do anything when I finished the race? What was the second place time? It would be cool to see a results screen after the race with your time (and your medal), the gold medal time, the silver medal time, and the bronze medal time. And then an option to replay! I think that would be a much bigger incentive for the user to replay stages.
It's pretty tricky to change lanes. I'm not sure you get a lot from being the user y position be a float. It might work better if the user snapped between lanes? It was just hard to judge when I was in or out of mud or grass, and stuff like the barriers that blocked two lanes seemed almost impossible to dodge.
You know this comment is coming, but sound! This really needs sound! The rev of the engine, different SFX for if you're in mud or grass, the cheer of the crowd!