Deliciously absurd. Not batshit crazy, just slightly out of sync enough with our expectations to be constantly funny. From the very beginning, the crime (forgetting his store loyalty card), to the very end, the tribunal sentence ([spoiler]that nobody could hear[/spoiler]), it's just a festival of funny situations.

Also the main point is that it's absurd, but only for us, it's not a hero finding himself in a strange situation that he doesn't understand. The situation is normal for him. Even if he didn't intend too, he committed a crime, he knows it, and from his point of view this is a real fugitive story. There's no wtf is happening to me moment, this is serious and he understands it.

Humour comes from a variety of sources, from small details (every single car being exactly the same), to recurring gags (Lozere is a totally abandoned place), serious (his awkwardly long declaration to his school crush), or stupid (people kicking him out of the car when hitchiking because he doesn't sing along). The memorable gags are too numerous to mention.