Military nicknames and callsigns often have double meanings, known only to insiders. "CHAOS" sounds pretty badass, and it is in the obvious sense.
But the real meaning was...
Colonel
Has
Another
Outstanding
Suggestion
Mattis is an innovator and experimenter, among other things, and the Chaos reference had to do with the frequent disruption of the routine caused by his ideas while commanding a maneuver unit. Generals are likely, even expected, to innovate but at the battalion or regiment level folks are more accustomed to simply executing published doctrine with vigor. They may not appreciate extra work in the interest of trying new things (in their defense, they have a lot to do as it is, and the military has special units which conduct experiments).