Notice that Bush doesn't crack a smile once. He doesn't smile or laugh because he knows Colbert is dissing on him for being a failure as a president or because he's just so dumb he can't understand any of the jokes. I'm gonna go with both.
Colbert has more bawlz than a bull elephant. Imagine if that were Bill Maher up there. Holy felgercarb. Georgie boy would be on fire.