All Ubuntu flavors use the same Ubuntu base that Ubuntu Desktop uses. The difference with flavors are the desktop itself, and the apps the desktop uses.
Ubuntu Desktop comes with 5 years of supported life; Ubuntu-MATE being a flavor comes with a shorter 3 years (though security updates continue the full 5 years for packages used by Ubuntu-MATE that are Ubuntu base packages, eg. Ubuntu-MATE 18.04 is end-of-life but if you have a system installed; a very large portion of it still receives security updates as those packages are common with Ubuntu 18.04 Desktop which is still supported).
To compare differences, you could always contrast what's found on the ISOs themselves; though'll you'll note much of it is identical (ie. both sharing the Ubuntu base).
Ubuntu MATE
Ubuntu Desktop