The assets are really well done. I especially like the (relatively) rigid adherence to the 4-tone green aesthetic of the Gameboy and the chiptune to go alongside it (did you use a tracker? It sounds great!) Aside from a few nitpicks (cubes accumulate on the death barrier in Level 3, enemies can't be killed between deaths), I really only have one major piece of constructive criticism - either the game is really slow or it's not well optimized for low-end hardware. I played through this on a Core m3-8100Y w/ Intel HD 615 integrated graphics and the game felt like it was running at roughly two-thirds - half speed. An options menu to disable certain post-processing effects would be a welcome addition if you continue to resume work on this game.
Good work!