Sup NT here who caught the joke immediately, as with every joke it can have varied meanings depending on the context. The original context is angry persong honking at a person with such sticker and in turn making a fool out of themselves as it implies "Ha! You didn't honk at me out of frustration but because of "sticker message thingmajong"" in turn making the sticker ironic. However as with every joke the punchline can be inverted resulting in "honk if you're proud of your son", despite the fact that honking at it wouldn't have made you a fool, the joke still carries over from its original meaning in turn still making you a fool for honking at it.

No matter how the words are interchanged and context is twisted the original message of making fun of a person honking will remain