Can my Nikkor 35mm f1.8 be used as a macro lens, with reversing ring?

Absolutely, however there are some caveats. You will have to move the subject because focus is fixed but there is a much better an inexpensive alternative; a macro focusing rail that mounts on your tripod. A Nikon BR-4 will allow you to view and focus while wide open and then close down to working aperture with a manual cable release.

We can do macro work with or without lens reversal. Why reverse the lens for micro work?

Most camera lenses are optimized to image objects at different distances and project their images on the flat surface of film or digital sensor. Most close-up objects we will image are flat or nearly flat surfaces. We are talking, copy work or the like. As a result, micro lenses are optimized to do flat-to-flat work.

A standard camera lens is optimized to project and image on a flat surface. We reverse a standard lens, pointing its rear towards the subject because this end is optimized to work a flat surface. Reversing generally does a better job when doing close-up work.

Additionally, the focal length of a lens is a measurement taken from a point called the rear nodal. The distance lens to subject is measured from the front model. Reversing the lens changes these points. Most often, the reversed lens, has a slightly different focal length that results in a little added magnification when doing macro work. That being said, the reversed lens works best on the typical close-up subject which is likely to be a flat subject.