Does the extension install at all? If it does, remove it and install all of the dependencies listed in the main page of the repository (scroll down to the Ubuntu section).
I just tried again on a separate fresh install and it works normally.
sudo apt install gir1.2-gtop-2.0 gir1.2-nm-1.0 gnome-system-monitor
Although I see that there's an issue reported from yesterday with the exact same error message. So it may be something else entirely. Keep an eye on that issues for any potential fixes from the developers.