CheckMK supports the idea of local checks. A local check is a simple script that runs in the agent on a host and performs whatever check processing and verification that’s required on the client end. This means you cannot customize the warn/crit thresholds from the CheckMK host end. But they’re easy to write.
The above simplistic script reads in the CPU temperature of the RPi, and sets a warn threshold of 90% of the throttling temperature with a critical threshold of 100% of the throttling temperature.
If you add this into;
On your Raspian install, then manually run check_mk_agent, you’ll see in the <<<local>>> section the output from the sensor. You can then edit the host in CheckMK and add the new service that is automatically inventoried. I assume here that your CPU die never gets below 0 degrees (should be fairly sensible in most circumstances, I imagine).