Garfield's "NO FEAR!" cheer, a proclamation of true grit normally applied to sporting contests and dangerous living, is incongruously applied to two guys who are going to eat as much gross pizza as possible. In the joke-logic world, the pizza toppings are so disgusting, or will render Garfield and Jon so smelly, that they actually will be a danger, and arguably gorging yourself on melted cheese is not the safest of dietary choices. This isn't actually far removed from normal Guy Behavior, where eating and drinking contests, and feats of consumption fuel good times and liver problems alike, all the time.
The slight difference is that Jon and Garfield, for no reason besides sheer contrarian mischief, decide to go for broke with their antisocial behavior. What else is there to do when your personal habits and desires — be it nasty pizza toppings, bad music, cigarettes, weird haircut — are being attacked? Even if social standards require only the smallest of personal compromise? Even if the reasons to cooperate are for entirely logical, understandable reasons? Garfield advises not only to let one's freak flag fly, but to crank up Here Comes Garfield and blow smoke in the face of oppression. Of course, this only applies when personal appetite is at stake. This is less War on Freedom than "Don't Crowd Me".
Show me the ultimate end-product of American individualism, and I'll show you a cat with garlic breath.