It's a good start, here's a few suggestions:
1. Use an AudioStreamPlayer instead of an AudioStreamPlayer2D node for your background music, having the background music panned in my left ear throughout the level isn't a really pleasant experience and I'm sure it isn't for other people too.
1.1 Your background music seems to be clipping too, try lower the volume on the master in the DAW you used and then use Audacity (free audio editing program) or Godot Engine (under inspector there should be something called Volume dB) to raise it back to a level that won't clip.
2. Objects could be placed in better positions, for example the yellow tree next to the text "Careful! A slimy enemy lies below!" looks a bit out of place.
3. Maybe add logic for killing the enemies (for example jumping on it or using a weapon of some sorts).
4. Adding the W and up arrow keys to jump would be nice. (It's easy enough to do, just go into your Project Settings > Input Map.
5. Adding more levels would be nice.
6. A main menu with settings (audio, changing inputs, fullscreen, etc.) would be a nice add to it.
Overall for your first game in Godot Engine looks good so far, keep at it!