So, since you just painted this, I'd suggest probably going a bit darker with the trees. When you have two separate masses with similar colors, they have a tendency to blend and distract the eye from the more important mass. It's a bit harder to focus on Squirtle and Bulbasaur, and to a lesser extent, Marill, because of this.
What you could do for ambiance, since there is a Zubat flying in the background, is turn the sky night-dark, which would give a stronger sense of tone to the main focus (the Pokemon sleeping) of the drawing.
I'm also a bit confused by the loss of one of Pikachu's feet, but I assume there was a likely a painting mistake around that area, which is perfectly fine, considering the tiny lines you had to work with - I'd hate to paint around that, too. I also noticed Bulbasaur's toes getting superseded by the tree - also something I imagine would have be difficult to paint over. I think what you could have done was penned over it after the paint dried - still can, as a matter of fact.
Everything else looks fine, so keep it up!
