Peak Gameboy-ness for sure! I especially like how you kept the shadows on the edges of the screen from the Gameboy itself, that's a great and accurate detail.
Your gore fill algorithm is really impressive! I like how mechanically it makes ledges that used to be safe for some aerial potshots slowly into unsafe areas, and then finally inaccessible. The way you calculate how body parts trickle downhill to fill in the lowest areas is fascinating, I especially like how the little gaps under platforms naturally form. I'd love to learn about your implementation.
Your movement mechanics are quite strong too, I like how momentum is carried in midair, and it's quite smart how you start the player falling and sliding down the ledge, to subtly prompt a wall jump.
I think gameplay-wise, you'd benefit a lot from more than one enemy type, especially one that moves faster. You might also want something that can either climb over or pass through walls: there were lots of times where I had corralled enemies into a small space where they were stuck. Having an enemy type that could escape those situations and put pressure on the player would go a long way towards keeping things interesting.
Keep up the great work!