Okay, so, here are a few tips to make your platformer game better 😉
1. When the player has to drop down from a platform, make sure he always can see where he will land.
2. Do not create deadly traps like the one with the 5 slimes or so. I dropped down, killed them all, and then was trapped and had to restart. Also goes hand in hand with tip#1 - I dropped down there in expectation of the level continuing there.
3. Do not rely on the player doing pixel-perfect jumps from platform to platform. It just never feels good.
This being said - I like the approach of this mario-style / family-friendly platformer. You've got a good start here.
Hope this feedback helps ✌