I really like the art style and this is impressive considering its the first game you've ever coded.
But wow the default Godot physics engine has quite a few issues... I recommend 2 things that might help:
1. Move a Camera2D instead of moving the platform up and down so that the creatures don't jitter up and down.
2. I noticed that when you start stacking heavy creatures on top of each other they can sometimes get pushed through the platform, so maybe make the collision box extend much further downwards and also a little more horizontally because it doesn't reach to those little yellow bits on the ends.