First off, great job on getting something finished despite the lack of time! I know Godot can be a tricky engine to work on sometimes since everything is done from complete scratch. Now, onto feedback.
1. I think the jump could have been fine-tuned a bit. Like, instead of having to press and hold the 'S' or 'Down' keys, you just release the Jump key and you drop.
2. For the boss I would have appreciated some kind of hit feedback even though there was no AI. I didn't even realise I was dealing damage to it until like 5 kicks in I noticed a red tinge to the character.
3. Something you know already but try dedicating a bit more time to the code instead of the art next time. I have issues with overscoping as well (I didn't even finish my submission...) but I usually try to dedicate more time to the core mechanics of my game first before the art. It doesn't matter how good your game looks if it can't deliver the full experience.
Good job overall, though! Incomplete or not, you still have something in here and experience gained.