Very cool idea, although not fitting the theme perfectly. I like the simplicity of the controls (mobile devices version with some improvements and more levels would be nice)
It would be cool if you added a button which showed "the solution" for levels that less experienced people (like me) get stuck on (I think it's more "educational" than all level unlock). A great QoL feature would be an indicator of your last sling on which you would base your next attempt (like in some Angry Birds games)
The level maker is a big plus to increase play time. The game has a nice style and calming music.
Well done!