Well within the state's rights power.
I also happen to approve. I don't relish explaining to a 4 year old what tasteless things some truck owners wish to impose on others.
Again, so long as there are "public" areas, the public has a say in what is acceptable there. Want to drive around with "Truck nuts" on private property? Knock yourself out.
On this logic, could the town ban the truck itself? Say, on the idea 'this is our hippie-lolberal town, and pickup trucks hurt mother Gaia'?
Could the town ban bumper stickers that say REPEAL PUBLIC NUDITY LAWS, DOWN WITH PURITANISM?
Again, obviously legally they might have issues, I'm asking for the moral viewpoint.