Heard doctor comment about my flat feet. Is flat feet pretty common?

Yes. Flat feet are common. Many people live there whole lives with flat feet and not have a problem.
Very common. Flat feet, or pes planus, is an extremely common condition. Most people who have flat feet since childhood generally do not have any problems, and never need any treatment. Children with severe pes planus, or significant pain, should be evaluated by a pediatric orthopaedic surgeon. People who develop flat feet as an adult should also be evaluated.