A lot of it just appears to be knowing how to create land curvature, particularly with edges. That, and just picking the right palettes and ExGFX additions can go a long ways towards making good overworlds.
I could certainly create something similar if I felt like it, but it just isn't a style I strive towards.
I think this is a cool idea for a hack! Playing as a Muncher plant! Also, the storyline is both logical and amusing in my mind, with its relation to floating/stacked Munchers being a common issue in many poor quality ROM hacks. I look forward to giving this hack a try!
I like both screenshots, but I find the first screenshot to be a bit weird, because the shadow is just on a part of the ground, and it's just in the left side of it. Nothing major though, just me being nitpicky as always.
Looking great so far! Overall design seems good, and the graphics are pretty cool as well (specially the bones). Really liking both the bones and the oil here - they help making those mountain levels more unique.
The cave dirt feels a bit odd to me, though. I think it's slightly more repetitive than the regular one due to Map16 stuff (see this for the original tiles).