The only unit I’ve seen for temperature sensors is “°C”.
C degrees and K degrees are the same units just 272 degrees apart for convenience
How did this solve the question?
Every time I use the value to calculate some stuffs (for examples, p=nRT/V), I need to subtract it by -273.15 first 
We definitely need a Kelvin readout for the System Monitor.
Otherwise, it increases the steps required to calculate the atmospheric pressures in our adiabatic cabinets.
For what it’s worth, I don’t see an existing feature request for this in the KDE Bugtracking System.
If you’re interested in submitting this as a wishlist item for Plasma developers (I believe this would be under the plasma-systemmonitor product), I’d recommend checking out the Community Wiki guide to getting setup and submitting tickets: Get Involved/Issue Reporting - KDE Community Wiki