Suddenly the "diagnosticd" daemon rise up to over 200% CPU usage Suddenly the diagnosticd daemon rise up to over 200% CPU usage, even with Xcode is not running, on OS X Mojave 10.14.1

I can't figure out it. The fans become very noisy.
I had the same issue and realized that some process (in my case osascript) was flooding the system log with error messages (AEGetDescData of non-desc type 'reco' not a good idea). This apparently led to lots of CPU intensive processing in the Console app (I see that's pretty high up on your process list as well) and the diagnosticd process .

Quitting the Console app solved the issue.