One thing to fix is the platforms. Don't use platforms that are only one tile wide, since there's not enough space to jump from them properly if you insist on using up as jump. Also, don't put them too close to the ground if you're going to make them solid on the bottom, as it may make it difficult to jump on enemies.

Besides that, I'd recommend putting a tutorial on that blank level at the start, since it took several seconds of palming the keyboard to figure out how to move.

I'm sorry but this really isn't all that good so far

For starters, the sprites are all poorly coded into the landscape. I would recommend making a texture pack with your own style to fit into the background a bit more or if you do decide to go online and get sprite sheets from there, please at least credit the original maker of those. A Metal Slug character jumping on living mushrooms in a Japanese landscape seems unfitting. The enemies die at times where you're standing still too which is weird

The collectables floating around the stage do nothing to increase your score (which there should be), upgrades, etc. For these, I would code them to open the exit when you collect them all in each level. Speaking of levels, the design is lacking and looks like they were pieced together in a level editor to where it's very easy to pass each one. The game is way too short in duration length and needs more of those to go through as well

There's also no sound effects either. It's complete silence whenever I progress through the game. I can message you some music to use for the game as well if you want, please let me know if you need help with that

Hope this review helped