Your game makes me realise how terrible I am at side-scrollers... haha
The colours are fantastic, sprites crisp and music well fitting--you really managed to built a very strong and coherent mood which is an incredibly hard thing to do!!
For me (after dying a lot) it may have been nice to have the level simply restart on death instead of having the menu come up, as that slowed down the game a lot for me.
I also do agree that the jump felt a little slack, making it hard to jump again after landing.
Also could be cool to see the tutorial be integrated into the gameplay in some way--say it could happen the same way but you get to try out pressing the buttons to get a feel for the game before it starts.
Look forward to seeing what comes next!