This is a really solid entry.
The character sprites are lively and the backgrounds are very well done! Sometimes there is a contrast problem between foreground and background elements, though.
The take on the theme is very interesting and gets some of the benefits of multi-player without the requirement of players being present. Over time it does feel less like each player is making a meaningful change to the level as much as they are just rearranging it to a random mess.
Accruing and testing changes is a nice system though since you can save for trickier sections.
Personally I don't find much issue with the controls because of the double jump offering a lot of extra control, though an indicator could be nice for the tutorial section.
The music was nice, I quite liked how it would dynamically change between editing and climbing.
Others have mentioned bugs which I also encountered though nothing game-breaking. I would recommend some kind of progress metric or just make it more clear that the game isn't endless.
Very enjoyable game.