I am using MX Linux 21.3 with Mate desktop. I have already installed mate-system-monitor (Mate System Monitor 1.24.1). This apparently does not give me the System Monitor plugin that I want for the panels if I choose "Add to Panel". Currently, it is missing from the list:
Actually, I do not know whether the plugin should be installed with the Mate System Monitor app itself. I have another Mate desktop using a different Linux distro. Its Mate System Monitor app is also at 1.24.1, and that one has a "System Monitor" in the plugin list for panels, like this (third in the visible part of the list):

