A knock sensor is basically a microphone bolted to the cylinder head that "listens" for predetonation. Damage to the plastic part of the sensor has no impact on its performance, so long as the electrical connection is intact.
Predetonation damages an engine, so if any is detected, the timing is retarded. It shouldn't have any effect on your fuel economy. I'm pretty sure you're looking at noise in your fuel logs, unrelated to your knock sensor.