Are you referring to the monitoring tab with various widgets you can display?
In ZD gui go to:
real time monitoring
choose start/stop and a time value (screenshot below)
I haven't heard of any particular values to be considered good because it will depend on how many access points and how many clients (for instance) you are supporting.
Bad would be 100% and time to consider an upgrade or find out why you ZD is stressed.
If your ZD responds sluggishly or feels "slow" then you know it's working hard and the monitoring tab will only give you pretty pictures to confirm this!
Rather like task manager in windows you get a feel for the numbers and what's normal rather than going by hard and fast rules.
NOTE: I seem to remember (from a comment by tech staff) that actually using the monitoring tool was quite cpu/memory intensive so do not leave it running in background unless you have a good reason.