Short answer is: no. The long answer is: Speed of light on glass is not so different than electromagnetic signal speed in copper, approximately 0.66C. Considering clock sources output electrical signals, using a coax cable instead of fibre optic has several advantages: simpler signal chain means lower delay, and avoiding highly noisy optical-electrical conversion means less jitter.

Fun fact: RF signals in air travel faster than light in glass fibre (0.8C instead of 0.66C).