I made the music for this game.
Hi, since you did the music, I'll try to focus on that as well.
Music
You did a really good job on the music! It all feels like it fits into the game and the atmospheres of the different settings.
The title music brings imminent energy and genuinely has a really catchy melody. Exactly what you would want for a game with such a fun and chaotic concept. The yard music is more toned down and calming. It feels thoughtful, complimenting how the player would be strategizing for the next race. The racing music is upbeat: feels like it's building up in anticipation for the finish line. And finally, the race completion. Heightened cheeriness which leads to a calm, silly, ragtime-y feeling. It contains the mood but acknowledges that the race is over.
All-in-all, the music was really good at encapsulating the feelings of all the environments, masterfully done.
Now for sound design as a whole: the game would really benefit from having sound effects. They don't have to overpower the music, but they can work hand-in-hand with it to make the game feel more alive. Menu buttons, environment, running, mutations, or anything else.
The rest of the game
I really like the gameplay: it reminds me of a mix of Umamusume and Duck Life.
The charming interfaces, music, character names, and pixel art really adds to the simple, cheery vibe. It makes me smile. Cheetah Champ is genuinely such a champ lol.
The buttons could be more interactive. Sound effects and motions are the answer. The motions don't have to be too fancy and could even be a static animation, but anything to make the game feel more interactive. The menus could subtly appear too rather than instantaneously. Motion helps bring attention to the fact that something changed. Sound effects reinforce it.
I also feel like the buttons should be consistent, and the Start Race button shouldn't be hidden under View Racers: it should be it's own button. Like the Experiment button turns into a Return button, so I expected that the View Racers button would do the same.
But still, really lovable game! I'd love to see what projects y'all work on in the future!