Pretty nice but got old really fast. Adding stuff like scoring, pickups and something along those lines would make it last longer.
Kinda weird how your most impressive looking thing in the game is the background. I mean you added animation to it and all. Those green enemy guys even had their own walking animation on the sprite sheet so it wouldn't have been too much work. And flipping them to face the correct direction would be simple too. The player sprite is very stiff too. It would be quite a lot you could achieve by just simple position/rotation/scale animations.
Moving and jumping overall feels too quick and sudden. Also if you mash the jump button quickly, you can trigger multiple jumps before leaving the ground for some impressive hops.
Anyways, good job!