In Slaughterhouse-Five, why is Cinderella referred to as "the most popular story ever told"?
Slaughterhouse-Five is a very pessimistic book, and also a satire; the tone of the book is almost unrelentingly dark (or funny, depending on the reader's sense of humor), and so its reference to Cinderella as "the most popular story ever told" is likely a self-referential dig at the novel's tone.
The actual reference comes in chapter 5:
And, at the far end of the shed, Billy saw pink arches with azure draperies hanging between them, and an enormous clock, and two golden thrones, and a bucket and a mop. It was in this setting that the evening's entertainment would take place, a musical version of Cinderella, the most popular story ever told.
(Vonnegut, Slaughterhouse-Five, Google Books)
Cinderella is, of course, an extremely optimistic story about a poor girl who becomes a princess. In a sense, it is the antithesis to Slaughterhouse-Five, which presents the world as a place where every event in history is set in stone and cannot be changed; it is outside of Billy's ability to change his fate, and he cannot rise beyond his historical destiny. In contrast, Cinderella is all about a person who utterly changes her position in life; it could be said that the citizens of Slaughterhouse-Five unconsciously know that they are incapable of changing their fates, and so embrace Cinderella as a reflection of the world they wished was real.