Yes, gravel is bad for cory catfish. While corys can survive with gravel substrate, they won’t thrive with it, as gravel interferes with their eating habits.
Cory catfish are bottom feeders, so they eat by sifting substrate through their gills. They aren’t able to do this with even fine gravel. Moreover, gravel can scratch their bodies and pinch the barbels in their mouth, leaving them injured.
Sand is the ideal substrate for corys because their natural habitat is sandy river bottoms. Soft river sand, natural black sand, and coal blasting sand are the best for cory catfish.