mate-system-monitor was forked from GNOME System Monitor in the early days of GNOME 3. It is maintained by the Mate Desktop team. The issue tracker is well managed and the Debian Mate Team quickly packages their stuff.
It doesn’t install that many mate dependencies - basically translation stuff.
Mate System Monitor does NOT play nice with Pocillo - however with a bit of tweaking (in git master of the pocillo-gtk-theme repo) the treeview headerbar is fixed in light styling and dark styling. Plays nice with Qogir and Arc theme. Materia does not work that nicely though.