That's just a bad analogy. That's more like if a comedian can only copy someone else's jokes, they're not funny. If you wanted a pizza analogy, it'd be more like if you can only make thick crust pizza, but can't make any other types of pizza's, you can't make pizza.

Which is just not true. You just find an audience who enjoys being constantly given thick crust pizza.

Mean/dark humor isn't an incorrect type of humor. It is a different type of humor. If someone is good at it then they just need to stick with that audience. Is it what you'd consider high class or in good taste? Maybe not, but it's still their humor.