Sullen impact

After almost six weeks of driving me back and forth to the hospital while I feverishly photograph oddities and infringements, Mick has become quite the decal/dickhead spotter.

As we approached home this morning he instructed me to look at the car behind us.
And there it was, the decal I'd missed earlier, this time on a windscreen:

What on earth possessed the owner to put that there?
And why "Sullen"?

Why not Moody?
Churlish?
Sour?
Unsociable?
Pram-faced?

Perhaps 'sullen', like 'wicked' and 'hectic', has taken on a new meaning among the younger generation:
"Hey bro, that's totally sullen."

I'm not so sure.