Extremely impressive considering this is a game jam entry made in only a few days. This is probably the best entry I've played so far and I'd be surprised if any future entries I play manage to beat it.
Aesthetics are easily 5 stars, sound design is nice, the procedural generation feels natural and everything meshes together really well. I'm having hard time trying to find a genuine criticism. I guess there could be more enemy and obstacle variety as well as more interesting procedural generation? It can also feel like you're screwed if there's a flying enemy right at the start and no bones for you to pick up to get past it. The camera also feels a bit snappy and jerky.
Apart from those nitpicks, this may well be the best game of the jam. Everything looks nice and I love the Wall of Flesh or rather "Floor of Flesh". Really good job! :)