i have a similar one for my syrian hammy Bandit Boi, my one has a flat bark roof and he's had no issue with it you can probably find my one by googling trixie flat roof corner hamster house. i tried attaching a picture but it wouldnt show for some reason lmao
from what i read having different rough textures in a hamster cage is great because it helps them keep their nails nice and short without you and your hammy having to stress about trimming them i dont think you have to worry about adding moss to the roof tho i think your hammy would enjoy the moss (assuming its clean and bug free) you could just add it in the cage somewhere for your lil guy to investigate but personally i'd leave the roof the way it is