The disclaimer has no legal foundation. I suppose if someone IS damaged, the owner of the truck can TRY to make such a defense that they had a warning sticker, but I doubt it will hold a heck of a lot of weight. A load is still required to be properly secured or covered and no manner of warning is going to absolve a person of liability. It might help mitigate some liability ... maybe ... but i do not think it will do too much.
A Nor Cal Cop Sergeant
"Make mine a double mocha ...
And a croissant!"
"To one who has faith, no explanation is necessary. To one without faith, no explanation is possible."
-- Thomas Aquinas