I think the physics are well tuned, and I could see some very cool courses being designed around the idea of using the gravity of planets to put spin or curve the shot of the ball.
I will echo the thoughts that a better sense of geography, and knowing the full layout of the course looks like would help a lot. I know there is a freeaim mode but I might be tempted to have the arrow on as a default and have it toggle on or off with a button press.
I also think having the camera pan across the course from the flag back to the player at the start might be good to help the player get a sense of what obstacles they need to avoid / where they need to go.
Classic gameboy color palette, solid idea and the feel of the physics and controls were all good. :)