Continuity mistake: When the bullies put the snake in Courtney's bag, it's brown. When Finley takes it out, it's changed to black, and is also a rubber snake. And when Courtney picks it up, it's real again, and is now yellow.
Revealing mistake: If you look closely, you can see human eyes inside of the mud monster's left eye. Pausing or slowing down is not required or necessary, but is easier to see if you do.
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.