The KDE Plasma Thermal Monitor widget has an annoying issue where the temperature sources will sometimes show as “Off” after rebooting. This seems to have broken around Ubuntu Groovy for me. This is a well known issue as referenced on the KDE forum among other places.
To fix, I edited
This line needed to be changed:
property var systemmonitorAvailableSources
property var systemmonitorAvailableSources: 
To fix with sed:
sed -i.bak 's/property var systemmonitorAvailableSources/property var systemmonitorAvailableSources: /' ~/.local/share/plasma/plasmoids/org.kde.thermalMonitor/contents/ui/main.qml