Search results from the first pages of results are usually optimized for mobile devices, but there are also pages better suited for desktop. After the first 2-3 pages of results, you'll find more and more results that don't have the mobile-friendly label.
Google's footer links to a page that offers more information about mobile-friendly pages. "This message shows under search results when Google thinks the site works well on a phone (mobile-friendly). The site works well because it doesn't make you zoom to see parts of the page, or it doesn't use features like Flash that you don't usually have on your device," informs Google.