GAZIANTEP, Turkey—Youssef Helali, who joined the Syrian uprising in its early days, has seen many of his one-time comrades embrace Islamic State, seduced by the extremist movement’s slick propaganda. So he found a way to fight back: making fun of a group that thrives on fear.

Together with fellow Syrian activists Maen Watfe and Muhammad Damlakhy, Mr. Helali has produced a series of comedy sketches that mock Islamic State’s pious message. Such slapstick can be a potent weapon: Islamic State, known by its Arabic acronym Daesh,...