It's possible you're thinking of a few things:
* Goosebumps: A popular horror fiction book series by R.L. Stine. It features various scary stories, but none explicitly feature a "Masked Mutant."
* Goosebumps (TV series): A TV adaptation of the book series, which also didn't feature a "Masked Mutant."
* A fan-made project: You might be thinking of a fan-created comic book or story. These exist online, but they wouldn't be official Goosebumps material.
If you can remember more details about the comic (e.g., the cover, any characters, the story), I might be able to help you find it.