Jr. is just a port of the original Monkey Ball, only with worse graphics, bad controls, and overall poor execution.
Graphics: Poor. Very poor. The graphics are very blocky, and you can't even see your monkey's face. Animations are crude and rough, not to mention slow.
Sound: All the sounds are there, reduced to portable fashion. Reduced, pretty badly.
Overall, this isn't really a game. It's just a broken port. Anyone looking for a good portable SMB experience should pick up Touch and Roll. But if you only have a GBA or SP, and you are NEVER getting a DS in your life, then, and only then should you pick it up. Otherwise, a bargain bin find.