I faced with that error on 12c database. The following error appear in the alert log:
ORA-12751: cpu time or run time policy violation

MMON consumes more CPU due to the monitoring activity in 12.1.This can happen due to Adaptive Optimization, a new feature in 12c.

The new feature can be disabled to reduce the CPU consumption:

SQL> alter system set “_report_capture_cycle_time”=0; /* Default is 60 seconds */

There is no negative impact in setting the above parameter as it disables only the automatic report capturing feature introduced in 12c. It does not disable the original SQL monitoring framework. SQL monitoring can be used very well without any issues.