The offerings in the KDE store for cpu temperature widgets are old and outdated, and they seemed to cause instability in plasma over time for me. Luckily there is a way to make your own that's stable! Download the "Command Output" widget by Zren from here or in "get new widgets"
Add the widget to your panel, and in the command in the widget settings, put:
"sensors | grep -A 0 'CPU' | cut -c16-21"
Note: You may have to adjust the command if the sensors output on your system is different, so experiment in the terminal first.