Music is fantastic, as a Super Monkey Ball enjoyer myself, the music is 33% pf the fun.
The hovering over buttons seems weird since I can hover over multiple buttons at once. I wish the text on the buttons could be more easily read aka darker than the button text when it's having the weird thing when it appears hovered twice. The character select should allow the big circle to be clicked than only the tiny one.
World 0 is hard, Monkey Ball has the first two worlds being joke simple worlds to teach the game's mechanics.
Levels are also very long at the start. One improvement would be for the banana system many monkey ball games have which has multiple bananas being gotten giving a free life, giving a reason to collect them.
I like how the game over in modern monkey ball shows the monkey in the ball sad and then getting very happy before being shot into space when selecting restart and acting more sad when trying to quit. Something like that would make the characters in the ball much more expressive so there is a real difference between the characters.
I would buy it once I get a portable PC for work breaks if you have controller support. Since I love monkey ball for switch and it would be too much of a stretch to request switch support at this stage of development, but you should consider it for the future.