The Mob is a group of people whom appeared in The Monster of Phineas-n-Ferbenstein. Constance tried to tell Mob about the platypus monster Dr. Phineastein and Ferbgor built. They took the place of Linda in this episode. Like Linda, they didn't come on time to see the platypus monster. They also are Jekyll Doofenshmirtz's inventions' judges.
It is also mentioned that Mob has a schedule for who or what they're gonna be "mobbing".