"Don't judge a book by its cover." is a bad idiom, because bookcovers are desinged to represent the content of the book.
"Don't judge a book by its cover." is a bad idiom, because bookcovers are desinged to represent the content of the book.
I think the point is that the cover is never guaranteed to accurately represent the book.
Quality of cover =/= quality of book
That’s one of the purposes of a cover, you could achieve it without any design effort.
But that’s not the point, not the main purpose of a book cover. Your previous poster is right, the cover is advertising the book.