Diameter of valve hole in the inner wall of the rim Let's say I have presta valve, so the outer hole in the rim is 6 mm (the visible hole). However rim can have inner "wall" inside with inner hole in it -- to my surprise the hole has bigger diameter.

Is it sign of low quality or is it norm around rim manufacturers?

My first reaction is "low quality" because bigger hole allows the tube to be rotated slightly, while the tight hole forces tube to be seated correctly with valve perpendicular to the rim.
The area at the bottom of the valve stem where the stem joins the inner tube has a larger diameter than the stem itself, and hence requires a larger hole in the inner wall of the rim.