This issue my also be relevant for other NetBSD architectures as well...
The Mate desktop lacks window frames and buttons after install. I tracked this down to a probable issue with the installer complaining that it wasn't possible to install osabi. After having tried to build osabi from source (version 9.3) and getting a mismatch from the other Mate packages (version 9.0), I tried building mate-control-center from source. Things were going fine, but taking some time and I go bored, so I started writing this post and was distracted when I was again, again, again, prompted for my root password by the build process, entered the wrong password and exited with an authentication error.
Sooo. I decided to ask here, only having been a NetBSD user for three days, what should I do next time around, to get the Mate desktop installed. Not because I am married to it, it could also have been XFCE (or Trinity if that is possible), but as I don't know if I will run into the same problem again, or how much I actually was ending up compiling, I thought that I would try to get some advice from more experienced users