Revealing mistake: The rocks and tree trunks bend and give way under the weight of Cooper, after he and Fergie are pushed into the cave in the ground.
Continuity mistake: The small twig in Fergie's hair disappears and reappears randomly, after her and Cooper are pushed into the cave in the ground.
Answer: Whoever created the Masked Mutant comics would still be around to create other comics even with the Masked Mutant dead and the Galloping Gazelle retiring, so Skipper could still receive different comics with all new characters. When Skipper goes into the Masked Mutant's lair, he is hit with a beam of light that turns him into a comic book character so after saying that he's the Colossal Elastic Boy, he turns into the hero at the end of the episode.