Trisquel reuses the packaging work from Debian and Ubuntu. If none of those
projects wishes to maintain a package for MATE, this desktop environment will
not be easily available for Trisquel. It may happen though.
Notice that:
your tastes on GNOME 3 are not universal: some people, including myself, like
GNOME 3;
Trisquel's configuration of the GNOME Shell (GNOME 3's interface) enables
icons on the desktop;
the Fedora project has worked on software rendering for GNOME Shell (no need
of 3D acceleration) and it will hopefully end up in Ubuntu and then Trisquel;
navigating between workspaces and windows can be pretty efficient if you know
the shortcuts;
GNOME Shell is fairly configurable through extensions that are a bliss to
install and uninstall
That said, if you want to install MATE, the process is indicated here
(Trisquel 5.5 Brigantia being based on Ubuntu 11.10 Oneiric Ocelot).
