just share,, if you need mate-menu for gtk2,, you can use my forked and modified mate-menu
I fork it from mate-menu on bitbucket at 5.7.0 tag
I modified it just to make it look simpler
https://github.com/mekatronik-achmadi/achmadi-menubut seeing last comment date,, I think you already solve the problem,, sorry
don't forget to add "dbus-update-activation-environment --systemd --all" to your bash init file,, so mate-menu can get all environment variables