It seems like the grass texture that goes on top of the grass terrain is missing from the map, makes the map terrain look pretty bland and boring

I think it is the decoratives that you are used to seeing. This mod does not place any (it's quite complex, but I've been looking into it myself), maybe a way for this mod to copy them before replacing the tiles, and replace them, altho you might get rock decorations over grassy terrain, and grass decorations over rocky terrain ... like I said, it's complex.

True, the mod only changed the underlying terrain, not the decorations. However, I might add this at some point, I'm tracking request here atm: https://github.com/TheOddler/FactorioWorld/issues/11

I currently regenerate the decorations, not sure how much difference it makes for grass, but it definitely adds a bunch of fish in the sea.