Ubuntu has supported the Raspberry Pi for many years, and in the past it has used server images with the desktop installed on top. Not ideal but it worked. I've used it in the past to install Ubuntu on Raspberry Pi 2 and 3. I even remember using Ubuntu Core to build a basic desktop machine.
Martin Wimpress from Canonical created Desktopify as a means to create an automated and well supported means to convert server images for desktop use.