I have a ubuntu server installation that I now wish to install MATE to. Is there any difference between reinstalling with a ubuntu mate iso or installing mate using tasksel?
There is a difference but you may not detect it. The basic Ubuntu packages are geared for Server so a fresh MATE install gets optimized for a desktop.
You can always give it a test run - let us know how it goes. Fresh install if any issues.
ubuntu-mate-core = very basic
ubuntu-mate-desktop = adds a bunch of apps (mostly)