main problem for me, is that you really only have one tile because you crafted the cliffs by using the floor tile and cutting away pixels till you got a cliff edge. Main problem with that is you don't have a lot of tile variation as a result, which makes things boring. Rather than make the cliffs using another tile, make it from scratch. Have a floor tile next to it so you can ensure the tile edges are seamless.
Your ceiling tiles would benefit from even moderate detailing. Check out super mario world 2 and see how they detail the ceiling compared with floors.
Last couple of things you might want to consider:
-more of a gradual transition from rock to flat dirt. Make it take two tiles to transition instead of one. Mostly suggesting as a way to make the rocks more interesting
-make a second floor tile, so you can have more variation