What does it mean to be a good community manager?
The shortest definition of community management is that it is a set of activities to make a community work efficiently. Community manager is an enabler, who activates users and makes them work together, again, efficiently.
So, I would say that efficiency of the community is a good proxy metric for the success of its community manager. Efficiency can be defined from the angle of community health or community performance. Community performance can be measured from the perspective of value created by the group or the pace of achieving the mission of the community. Community health is usually measured by the influx and outflow of active users.
Of course, there are a lot of “but” and nuances, but I would say that the above is the general skeleton that provides the good foundation of measuring the success of a community manager or a community management team.