I think you are improperly comparing community consensus with tragedy of the commons. Community consensus is more akin to regulation than it is to nash equilibria. It's the nash equilibria that has been a bad heuristic for finding the global happiness maxima, because it's through the nash equilibria that the tragedy of the commons occurs. In contrast, effective regulation can better ensure the global happiness maxima. So that's why community dialogue and a rough consensus of mores can be useful and productive towards the health of a society.