This is more of a curiosity than anything, but would Mate be considered the "flagship" Mate distro? I have always thought of it that way. Although looking into the history it seems like it started as a fork specifically for Mint (?) Who now focus primarily on Cinnamon.
GNOME/Fedora, KDE/Neon, Mate/(?).
Or actually another thing I also wonder - which distro do all the actually Mate developers use?
As for me - yes - the Ubuntu MATE is a flagship MATE distro because:
It is based on Debian, so we have stable and simple APT under the hood
We have LTS releases, so can live without worries for 3-5 years and jump to newer LTS releases.
The MATE packages are tested first on Debian and then came to Ubuntu
The MATE 
and Ubuntu MATE 
teams are healthy and they share ideas of traditional but fully-functional desktop demanding current needs
About my personal usage and experience - I'm using Ubuntu MATE on about 20 personal and work laptops and computers, have set up LTSP classrooms with 150+ computers running Ubuntu MATE.
Most of the developers use Ubuntu MATE from what I know, at least one uses Fedora and maybe some uses Mint
Wow thanks for this comprehensive answer! It does make sense that you would want to work on a stable base. Mate isn't necessarily trying to be the bleeding edge of anything, just add nice newer stuff when its ready and works great. Ubuntu Mate really does make sure to add some really nice user experience improving bits compared to some other Mate implementations.
Love all those installs you've done too! As for me, I've only got Ubuntu Mate on 4 computers 
Not sure what would constitute a flagship distro, but as you note, Mint had it before Ubuntu. Wikipedia has this to say:
"The project is supported by Ubuntu MATE lead developer Martin Wimpress and by the Linux Mint development team."
So, looks like both Ubuntu and Mint are primary contributors.
True, but on the Team section of the Mate Desktop official site it lists Clement as the founder in the "We’d like to acknowledge the efforts of the following people who helped bootstrap MATE in the early days" section, which sort of implies that he has sort of taken a back seat now with Cinnamon being Mint's main focus. It seems like a lot of the nice changes for GTK3 and with Marco were piloted once Ubuntu Mate took center stage with the desktop starting around 15.04 (?)