If you’re exporting an ePub, the book cover will show up in the ‘library’ when it’s uploaded onto a device, though it doesn’t show up as a separate first page in the book itself.
For print books, all publishers require you to upload a separate file that is specially designed to include a spine and back cover, so Atticus doesn’t export the cover at all for a print version.
If you’d like the cover to show up as a title page, you can insert a full page image, drag it into the first page location at the top of your front matter, and then it’ll export with your book.