“Fanfiction is lazy and derivative.”
Are impersonations lazy and derivative too, then? It’s hard to achieve a believable impersonation (impression). And when someone achieves one, the audience doesn’t say, “Oh, you’re just ripping off the person you’re impersonating,” because everyone is already expected to understand that the likeness is deliberate. A significant portion of the skill is in the ability to achieve the likeness. The likeness (and often exaggerating that likeness) is largely the point.
It’s relatively easy to create lazy, derivative work in the realm of original fiction, because the presumption of originality becomes a smokescreen to hide that the author is ripping someone off, whether fraudulently or accidentally. Conversely, in fanfiction, as in impersonations, the artist is immediately up-front about the work being impersonated, and in so doing sets the bar rather high. It’s important to remember, also, that there is a lot more to impersonation than simply mimicking the contents of a video clip of the impersonatee. A good impersonator internalizes the character behind the persona so thoroughly that he can convincingly involve the character in new situations and confront the character with new ideas.
A fanfiction writer who seeks to convincingly impersonate an existing fictional universe and its narrative style, especially in order to explore its unanswered questions by introducing new elements, is not being any more lazy or derivative than an actor who specializes in impersonations.