Harlan Ellison loves to quote Sturgeon's Law that 90% of everything is crap. Mental Floss (a magazine of "fun facts" that surely belongs in that percentile) dug up what it claims is the source of the actual quote:
Science fiction author Theodore Sturgeon wrote a defense of sci-fi* in the March 1958 issue of the sci-fi* magazine Venture. He wrote, in part...
I repeat Sturgeon’s Revelation, which was wrung out of me after twenty years of wearying defense of science fiction against attacks of people who used the worst examples of the field for ammunition, and whose conclusion was that ninety percent of SF is crud. Using the same standards that categorize 90% of science fiction as trash, crud, or crap, it can be argued that 90% of film, literature, consumer goods, etc. are crap. In other words, the claim (or fact) that 90% of science fiction is crap is ultimately uninformative, because science fiction conforms to the same trends of quality as all other art forms.
*The true heads of the '50s and '60s called it "sf," loathed "sci-fi" as trivializing, and must have vomited over "SyFy." Mental Floss claims (without source) that Sturgeon was the model for Kurt Vonnegut's Kilgore Trout, which is pretty cruel if there's a shred of truth to it. Sturgeon being one of the writers who helped raise sf out of its literary ghetto, not through ukases such as the above but through superlative writing, without ever abandoning the field that also gave us Vonnegut. Have always assumed that Trout was an avatar of Vonnegut's self-loathing and will continue to think it, with or without Mental Flossing.
The most devastating critique of sf to me is that it's "men's literature" in the same way Harlequin romances and other bodice-rippers are women's.