Mush is a squishy substance, generally incapable of sustaining life, let alone coherent thoughts.
Heads contain a lot of things like bones, cartiledge, muscle, and brains.
However, many brains have become mush, and since brains normally occupy the largest part of the skull, you can properly say the head is filled with mush. Human beings were designed by the Creator with an enormous number of reflex actions, so a head filled with mush may still exhibit some seemingly rational behavior. Then your head is filled with mush. I am in college. In fact, it is almost probable.