James is hardly that. He obviously cares very strongly about many things, otherwise why is he risking his life and freedom to fight Voldemort? He may appear laid back and casual aged fifteen, but that is clearly not who he was when he left school at eighteen. And can you see Dumbledore chosing a Head Boy who is laid back and doesn't care about anything?
As an adult, I picture James with a Ludo Bagman type personality. You know, not caring about anything, laid back, etc.
True, the prequel does show him laughing and teasing those policemen, but as soon as the bad guys turn up again, he's deadly serious and ready to fight. He knows when it's time to be funny, and when not to be. Even without the threat of Voldemort, I think James would realise that there were times when you had to be serious, and finding a job would be one of those time. So if sitting at a desk writing papers paid the bills that needed paying, or set him on the road to his dream job, he'd do it. But I can't see him actively chosing a job that was pencil pushing just for the hell of it.
If you've read J.K Rowlings prequel, you'll know that James was still immature (yet funny) when he was older, so I can't see him sitting at a desk writing papers, can you?