Does anyone actually take Lovecraft or Mythos fiction as fact?
This is a loaded question. To answer it, I will assume that Lovecraft (et al) created the core of the Mythos, as well as numerous tomes, but mixed in a generous helping of real-life people, places, and things. As an example, “The Golden Bough” is a real book (in fact I own a copy of this, IMHO, boring text), but the Necronomicon is not. So, though Lovecraft (et al) used “The Golden Bough” in their stories, this answer has nothing to do with whether or not anyone believes that the book exists. It does deal with those who believe that things like the Necronomicon exist, (as well as any ficticious race, place, tome, etc). To move on to the answer: Yes. Some folks take this fiction as ‘fact’.
