Gordon's advice is absolutely correct.
Upgrading or a fresh install timewise isn't that big of a difference in my opinion though.
You could also check lah7's post here if you have a separate /home partition.
This would give you a fresh Mate 20.04 install with just the Mate packages.
Installing a metapackage (like ubuntu-mate-desktop is) won't remove your old flavor(s) of Ubuntu (and it's package choices/config files as far as I know) but will install all the packages associated with Mate.
So if you're now using Chrome as browser you will get Firefox additionally installed (to set a simple example). Same for Evolution vs Thunderbird etc...
Just type
apt-cache depends ubuntu-mate-desktop
to see what would get installed.
Or just make sure to pay close attention to the output of the command gordon posted before hitting that Y button 
Really depends on what you want to achieve exactly. A totally clean Mate 20.04 install or a complete Mate 20.04 install on top of what you already have running now.