He doesn't need a mask, he has no secret identity. The only heroes with public identities who still need a mask are anyone using Iron Man or Ant-Man tech (due to their HUDs and communication devices built into the helmets - though Janet just uses an earpiece usually), anyone using breathing apparatus, and anyone whose power source is the mask (i.e. Nova, Rich was outed in the first Civil War due to his New Warriors connection, but obviously still has to wear the helmet). That doesn't apply to Hawkeye.