Nair does not make hair grow back thicker. There is a misconception that shaving or using hair removal creams causes hair to grow back thicker, but hair that is allowed to regrow comes back same texture and thickness as the original hair.
The misconception that using Nair causes hair to grow thicker comes from the regrowth process. As hair breaks through the surface of the skin, it creates stubble that looks darker and feels thicker than the original hair. However, there is no change to the composition of the hair.
There is no difference in how hair regrows whether using a razor or a hair removal cream. The only difference is that a razors cut the hair at the surface and hair removal creams melt it away.