There are a number of events which manipulate McClellan, but non of them actually evaluate his performance, and some of them do not currently work.
In Early October '61, an event fires which checks if the Union has take Manassas. This event dissolves the Northeastern Virginia Army, removes whichever models of McDowell and McClellan are in play and creates a stack named "Army of the Potomac" with ***McDowell and the ***McClellan model with Polit. 75, which should convince the player to create an army with McClellan or face dire consequences from the political penalty of putting McDowell in charge of the AoP. However, this event will always fail the way it is currently written. I've already asked Pocus if this is WAD.
In Late October '61, if the Union still does not control Manassas, McClellan is made CiC of the Army, his model is replaced with the one with Polit. level 100.
In Early March '62 if is CiC will be replaced again with a model with Polit. level 50.
In Early November '62, if he is at Polit. level 50 he will be demoted from CiC an replaced with a model with Polit. level 25.
In '64 he will be removed from the game because he is campaigning for the presidency.
None of these events actually evaluate McClellan's performance in anyway.