very unique concept!! it's pretty ugly but you've got some pretty thought out sounds and transitions to go along with it. the map of japan was a simple but effective frame of reference to ground the player and allow them to track their progress through the game. it definitely feels like you've got experience with game jams that show in details like that. the option to play with friends too was pretty neat!
the cutout bobble heads were fun but it'd look even better with more cutout style assets and rougher lines around the flat shapes. still, i was impressed with the use of sound with the ringing at the start of a level (it ringringrings when you spam the retry button) and the noise when you lose health having a nice oomph to it. i'd suggest pitch shifting that one ringing sound so it's less gratingly repetitive and to add some extra character to the monkeys. the transitions were cool but an option to skip the ones at the start of each level would be good.
i could be wrong but i felt the bikes had way more power than they needed. i overlaunched myself so often which might've been part of the challenge, but idk this game doesn't feel like i should be holding back as much as i have to. i think more space for the bikes to launch over between the ramps and platforms would feel more epic and right, at least for variety in one of the levels.
i think i was able to cheese the game a bit by landing my bike near the edge so that the other monkey had to slide off. which felt smart and cool of me but i didn't get to experience as much of the bike stacking as was probably intended. i can see how that can get pretty tight though, and it's an interesting use of the bike's shape. it could be more interesting if the bikes weren't completely immovable after being placed though, so maybe you could have like a tumbling tower moment.
with the freedom to move around the end screen i particularly felt untapped potential with the physics of that bike. like you could go soooo fast and do crazy maneuvers in the air that look fun to mess around with for actual gameplay. there's definitely more that could be expanded on with these foundations, like i imagine being able to push each other around would be fun, or more shape formations not just the one platform that could help structure or disrupt the stacks.