Very neat game! The concept is very intuitive and the graphics are incredibly pretty. They remind me of some very charming DOS games from before I was born, and the backgrounds especially are very funky and just abstract enough for my imagination to run wild, without making it hard to tell game elements apart.
The challenge is fair. It gets difficult pretty quickly, but I kind of enjoy that in game jam games, at least when they have several levels. That way you can have an easy first level to show the game off and teach the mechanics, but you can also really experiment with all of your mechanics as much as possible, showing the player that "this is how complex you can get with these mechanics", and they can get a taste of what the later levels would be like if it was a full-length game.
As for the controls, they are definitely responsive enough, but I think there are just a couple of small changes that would make them feel a whole lot more fun to use. Specifically, the characters feel just slightly too slippery, and the hitboxes for the obstacles could be made smaller. It seems that they are the same size as the sprite itself, but because it's so common in 2D platformers to make dangerous hitboxes much smaller, it tends to feel like you hit something when you shouldn't have (mainly the spikes).
All in all, the core gameplay concept is a lot of fun and the game looks very competent visually. With just some minor tweaking to the movement and collision I think this game would be incredibly fun to play even for a hundred levels!