I recently bought a book which spoke to me by its cover and it was one of the best books I've read in ages. And I still love the cover almost as I love the book.

But then there are books where I really disliked the cover but they are still great to have and full of useful information. (Most of these are non-fiction..)

I think the idiom misses the mark: judging is just one part of it. Being aware that lot of your judgments are going to be wrong, especially if you use only one source of information -- that is much more useful thing to keep in mind.

However, adages are (like) memes---the best ones don't always win.