Chriseindt"Don't judge a book by its cover." is a bad idiom, because bookcovers are desinged to represent the content of the book.
RhynoplazI’m going to say it’s like a chicken and egg scenario.
It’s recommended that you don’t judge a thing or a person based on a quick glance. That’s good advice.
Book covers are designed specifically to be judged at a quick glance because they know that’s what people do, despite the advice they were given.
I think it’s also true in reverse. Like if the book cover looks really cool but the story is hot garbage.