If you want to put a larger square sticker like that on the curved surface of a helmet, you can carefully make a few cuts around the edges of the sticker and they will overlap and make the sticker lay flat.
I have an MIA POW "You are not forgotten" sticker and a "Vietnam Veteran" sticker. Other helmet has "If you can remember the 60's you weren't there."
Did you ever see the black tee shirt on a rider with white letters on the back that say "If you can read this the pregnant dog fell off."