MCMgr is a singleton that manages job queues for the entire game[check]. It implements the gsys::ISystemTaskCallback interface and it is called from the gsys system task several times in postCalc.

MCMgr
Subsystem
Official name Yes
Description Manages job queues for the entire game[check]
Init function Switch 1.5.0: 0000007100DCB390
Wii U 1.5.0: ???
Debug only No

Considering that MCMgr's callback member function is invoked from gsys, it is possible that MCMgr stands for Model Calc Manager and that it was originally designed to only handle graphics tasks.