Here is a complaint on an NYT blog about car decals that say what beaches you head to for vacation, like the OBX one that stands for Outer Banks, North Carolina. (I don't know if these exist outside the East Coast.) Other prohibited decals are those that advertise what sports your kids play, including cheerleading -- things that might make them popular -- and decals that show your family togetherness by giving each member their own stick figure decal.
Are there any decals that the complainer would allow? Sure: the ones that signal where your kids went to college, ones that signal your political views, and presumably an Apple logo (hey, the personal is political).
Basically, being a status-obsessed dork is OK, while having a life is forbidden.