Yes, nylon bolts can fatigue if overstressed, like everything else. I would be more concerned with the wooden structure of the airframe though. Never over tighten. That said, nylon bolts (1/4"x20) are very strong. Do a simple test and try to break one. Nylon does get brittle over time, usually years, long after the plane itself has expired. Some of the cheap plastic bolts from ARF's I personally would never use. I replace these with true nylon bolts from Micro Fasteners, very cheap in lots of 100.

Safe Flying!