Well, pretend for a moment you don't even know Okami is a videogame. ... Okay, yeah, silly, but the point is, it's a big Japanese style flaming wolf that's not done toony or whatnot, and has the name "Amaterasu" across the windshield. If you had no clue it was a game, you'd just figure the guy had some Japanese styling going on.
I had a trainer at a job once who had the Terran Empire tattoo from the original Mirror Universe episode of Star Trek on his arm. He said he got compliments from non-nerds on it all the time... they didn't know it was a nerd tattoo, because they didn't watch friggin' Star Trek. They just saw some neat geometric shapes pierced by a wicked broadsword.
If you're going to get a nerd tattoo, or nerd out your car, you could do worse than stuff like evil empire insignias or wolves wreathed in flame, because those have crossover appeal. The generic naked blue furry chick on the back window, though... one look at that and the first thought's gonna be "WTF, nerd".
