Possible error on https://wiki.debian.org/Mate
Yes, it is a wiki. But I'd be doing no favor if ... ;/
Under the heading
To install MATE, do the following:
MATE just core
"apt-get install mate-desktop-environment-core"
When I installed Jessie I found it necessary to include xorg.
[The wiki page had said "If no graphical environment was present
before, you will also need to install a display manager such as
GDM or LightDM.]
The relevant section of my preseed.cfg was:
### Package selection
tasksel tasksel/first multiselect standard
d-i pkgsel/include string mate-desktop-environment-core xorg
xterm lightdm pluma
d-i pkgsel/upgrade select none