I am not to familiar with ArcGIS Monitor, and have never used it, but from what I've read about its function and see in your images, it is clear any activity tracked by ArcGIS Monitor is directly affected by your user's activity. You can even see the 12:00 to 13:00 lunch dip in the CPU usage of Monitor...
Personally, I wouldn't worry to much about 80-100% cpu usage, unless it directly impacts the usage of Monitor (slow updating, web interface no longer accessible due to time-outs or so, or whatever else blocks you from using it properly).
Instead, having 80-90% of your computing power sitting idle doing nothing is a waste of compute resources and ultimately money. Computers are designed to do work, not to sit idle, and servers and professional workstations are designed to withstand 100% CPU usage for essentially indefinitely. I routinely process global OpenStreetMap data on an HP Z840 workstation with dual CPU's (2x22 cores), which is essentially server hardware packaged in a very large desktop case. I trash it with 100% CPU for days to process the global OpenStreetMap data. Never fails on hardware.
Of course, the high CPU usage may indicate this server is at its limit doing real work, maybe because it needs to monitor a very large enterprise deployment with many users and servers and handle all the traffic for that, and you may need to anticipate a server upgrade if you intend to use it for an even larger enterprise deployment in the future.
