This is both really well designed and really impressive that it's implemented for the GameBoy!
If I had to critique one thing it's that the fact that you basically want to permanently mash B, the game gets a lot easier if you remap the emulator controls so that you can shoot with the same hand you use for steering, so the other hand can just permanently mash (e.g. WASD, Space for A, J for B). Before I did that, I usually died around 10-15 ghosts with a high score of 22. After changing it, I promptly got 48. This isn't an issue when actually playing on the GameBoy, but since constantly mashing a button is still going to be an optimal strategy, maybe it would be better to limit how often you can switch ghosts (or have it happen automatically) but then spawn fewer ghosts.
Nevertheless, full score for this one! :)
Btw, I really like that the ghosts prefer diagonal movement. Games that don't do that but only have orthogonal shooting tend to be really annoying.