Which model? This one? https://everymac.com/systems/apple/mac_mini/specs/mac-mini-core-2-duo-2.0-specs.html
You really need to look at the temp sensors for the CPU and drives.
Safe temp max for the T7200 looks to be near or over 80C so if the air temp is only 55C, I don't see the problem.
I have a rather aged Mac Mini (2008 ish I think) that I am now running a version of LInux on (too many apps just did not like the old version of OSX, so gave up on it).
But I am a bit bothered that maybe these distros do not handle the cooling side of the hardware properly. I know the internal fan is running at minimum, but it never changes. And the exhaust air is sitting at around 55degC.
So my questions is; is this a normal kind of temp for this box? Or should I be looking at some app to manage the fan better?