it looks great however I would just like suggest that you make things a little more defined. the art style isn't my thing but I know that even making something like this is hard so I give you credit however somethings stand out like one of the plants has an outline and the others don't which is a little jarring for my eyes or it might just be the colour pallet. here I have the tiles in the colour pallet I use. this is a great tile set and I LOVE the backdrop you provided.

Looks great! If it's public domain, am I allowed to make a derivative of your sky background which fits my game?

Well I know the answer to that, it's yes. It's just that you have applied multiple licences to the submission.