Magnetic pickups work by creating a magnetic field in which a string made of a magnetically sensitive material vibrates. Nylon is not a magnetically sensitive material, and therefore doesn’t give very good results with magnetic pickups. However, there are many great acoustic guitar pickups that use piezo under-saddle pickups, in-guitar microphones, active preamplifiers, and other systems that can amplify a nylon-string acoustic in a natural and pleasing way.