This isn't unusual. Remember that you're looking through the lens at its maximum aperture, and light falloff (aka optical vignetting) is at its worst at maximum aperture. (Physical vignetting is where the corners of the image are being physically blocked by the lens design, filters, hoods, or the like, but that's not the problem that you are experiencing.)

Macro lenses are almost never used at maximum aperture because they're designed to shoot macro subjects at very close distances. This sort of work requires the best depth-of-field possible, so macro lenses tend to be used at moderate apertures like f/8 at a minimum, and often even smaller apertures. If you want narrow depth of field, you will get it automatically from shooting macro subjects, and stopping down the lens even a stop or two is going to dramatically reduce the light falloff you're seeing in the viewfinder.

Also note that light falloff records on film and digital a little less severely than you see it in the viewfinder in some cases, so take some test shots before you dismiss it as being too excessive for your purposes.

If you want to shoot at f/3.5, I'd strongly suggest you get a traditional Nikkor 50mm lens (the 50/1.4, /1.8 and /2 are all quite good; they all have some light falloff but it largely disappears after stopping down one stop, so e.g. with the 50/1.4 you could shoot at f/2 with almost no visible light falloff).