i wish i had gotten to this before the ratings ended. this was very fun overall and i think your sprites are really cute! definitely has a GameBoy feel to it. i love the music and sound effects! the levels were nicely designed and you have spike hazards and a couple of enemies to add a bit of variety to the gameplay. i largely agree with everything danwithfaintpraise mentioned.
it would be nice if the stationary enemies had some kind of sprite change to telegraph when they are about to pop up and fire. you can figure out the timing easily enough, but it's a touch that really can help players and make the game feel more fair.
the jump needs some work, and makes the game a little bit harder or more frustrating to play than it should be. i think most players will agree that a platformer with a good arch to the jump just feels better. so, like Dan mentioned, you want the jump speed to gradually diminish as the player reaches the peak of their jump and then let gravity kick in and take them back down to the ground. even though it's using Unity, i think this video explains things pretty well. i know there are some great jump tutorials out there for GameMaker as well, so you might want to check out some of those. i think this HeartBeast tut covers it.
also because your jump goes up and then back down so quickly, it makes it virtually impossible to jump over the enemies. maybe that's something you wanted in the game, but it was a little frustrating for me. having a jump that has a little more air-time would also solve this issue.
i like the boss a lot. it has a nice attack pattern. i found the bullets to be a bit difficult to dodge due to the jump. i think they wouldn't be an issue if you changed it a bit. i also agree a little more feedback on when the boss is getting hurt would help. i wasn't sure if the face change meant i was hurting him at first, but that's great start. if you look at other retro games, often times they will add some kind of flashing on the sprite when they're damaged, or an impact effect of some sort. also a sound effect for when there's actual damage could help as well. i realize this is a jam, so you may have just run out of time for these things. i didn't get any sounds into my game, lol!
i'm still using GM1.4, but i assume version 2 has about the same options when exporting. when you go to "create application" to export there a few options to "save as type" below the file name. i would definitely agree to not do the installer.
anyway, sorry for the wall of text! i hope this is helpful feedback for you!