Why does the preview look blurry?

Two different things can be happening:

• The on-screen preview looks a little soft, but the export is sharp. The editor preview is scaled to fit your screen, so it can look less crisp than the real thing. Download the (free, watermarked) PDF and view it at 100% — if that's sharp, you're fine.
• The exported PDF is also blurry. Then your front-cover image is too low-resolution for print. Zooming a small image up to cover size can't add detail. Replace it with higher-resolution art — see What resolution should my artwork be? The Cover Maker warns you when a loaded image is under-sized for your trim.

Always judge final quality from the exported PDF at 100%, not the editor thumbnail.