I enjoy beautiful book covers: I present images of them sometimes on my tumblelog, and I subscribe to the RSS feed for the delightful Book Cover Archive. But we — I — should keep in mind this thought from Lynne Sharon Schwartz, who meditates on books she owns but hasn’t read:
Others were just too gorgeously packaged to resist. Book jackets nowadays have become an art form, and browsing through a bookstore is a feast for the eyes. In some cases the jacket turns out to be the best thing about the book. I am not one to snub beauty, wherever it turns up. Yet I have come to distrust book jackets calculated to prick desire like a Bloomingdale’s window, as if you could wear what you read. The great French novels used to come in plain shiny yellow jackets, and the drab Modern Library uniforms hid the most lavish loot.
(From Ruined by Reading.)
I don't know. A lot of book covers are meh, but there aren't that many that are actually unpleasant and bad. I've usually found that the best books may or may not have the best covers, but the best covers more often than not come on very good books.