What is the comparative roughness of fabric surfaces?
It varies with the level of finishing effort, and depends mostly on how much the weave of the fabric was filled in.  If the weave shows in the fabric, you can expect the surface roughness to be similar to lacquered wood.  If there are enough dope coats to fully fill the fabric, you'd have something similar to commercial metal painted finish.  But if you really go to town you can get to the filled and polished glass surface finish.  You'll see some award winning homebuilts at Oshkosh with a fabric finish that looks like shiny mylar plastic, but is actually regular fabric and dope.   Many many coats, and many many hours of sanding and rubbing.

Some of the latest finishing systems can produce that finish without zillions of coats and hours of rubbing.

This is all notwithstanding all the surface irregularities you have with fabric due to the use of reinforcing tapes etc., which may make the whole issue of surface smoothness moot anyway.