I like this premise for a visual novel where you are a street racer, never saw something with such themes before on itch.

The problem is the concepts of racing and the female-muscle-growth (fmg) aren't linked together in any way, not even a narrative of how fmg works in this world or why Daisy is doing these races. A suggestion for the author to link the two concepts together would be to add a supernatural element where the more the women upgrade their cars, the more they experience the FMG to make the setting the more interesting; this would also provide an incentive for the player to utilize the upgrade and buy parts currently in the game for the car.

You can give Daisy a reasoning her to participate in the races (since I don't think there is a reason for her to do this currently). Maybe a backstory or something where she had a sister who was a street racer but the top street racer (her being the top is showcased with this character being very jacked from her own fmg) raced her and in the race the sister died or something, so Daisy wants to beat the top racer and avenge her sister or something.

In terms of the actual races, the opponents you go against should be a bit more fleshed out (maybe some are friendly and just racing for fun, some are obsessed with being the best in races, some are just here to easily experience fmg, etc.). Heck even maybe give Daisy a rival that is consistent throughout the story and is a consistent threat to Daisy's racing career, with the rival getting equally strong or stronger as Daisy in terms of fmg due to the rival's own racing achievements.

I realize these changes would require the author to make the game less of a  management/clicker game and more of an actual visual novel with a heavy emphasis on story and dialogue but I think similar changes I mentioned above would interest more potential players.

If the author wants to be ambitious (which I wouldn't recommend until after they develop the game more) they can introduce variations in how Daisy's rise to the top as the best racer be by allowing alternate/splitting routes and choices that affects Daisy's personality;

if the player makes Daisy act more of mean, cocky or like a jerk, maybe she'll get along better with her rival and they end up become friends or lovers due to their now similar personalities with the consequence of Daisy ending up ostracizing most other characters,

if she becomes good she ends up going to the police and they end up helping her with taking down this whole illegal street racing operation in the town to prevent people from dying (like her hypothetical sister in this situation),

if you make her do criminal or violent acts, she'll try to sabotage her competition before races or frame people in her way to get to the top, throwing away her morality or principles to win no matter what. 

Also in terms of the actual gameplay, if it's going to be a clicker I would like the option to use the keyboard (either space or enter) instead of having to use mouse click since I think it would be easier for some people.