Cover Art is critical. In general most people will never read your blurb, look at your reviews, or open to look at the sample unless the cover grabs them first.
The cover's job is used up in the first 5 seconds or less of the sale, but its job is to tell the reader "this is a scifi" or "this is a epic fantasy" "or "this is a book about cats" or what ever. It needs to look professional and fit the genre.
Unless you have a name like Stephen King, your book cover is one of the most important parts of selling your book. If your name is Stephen King, just put that on a blank cover and it will sell fine.
