Ubuntu MATE and Lubuntu are unlike the other flavours. When installing the ubuntu-mate-core or ubuntu-mate-desktop meta package you must do so with --no-install-recommends, like this:
For a minimal install
sudo apt-get install --no-install-recommends ubuntu-mate-core^
For a complete install
sudo apt-get install --no-install-recommends ubuntu-mate-core^
sudo apt-get install --no-install-recommends ubuntu-mate-desktop^
It is required to use --no-install-recommends otherwise a whole heap of unrelated packages will also get installed.