Do you think it would be funny if I wrote 'wash me' on a truck that's dirty?
I don't see why not, it seems like a harmless enough prank.
I think it would be a funny way to tell the owner to wash their truck!
I think that this type of prank is appropriate and you would get a smile out of someone with it.
I think it's funny, so I'm going to write the message on the truck.
I agree, the point of a prank is for both parties to laugh and I believe your harmless jest is in good taste. Making others smile is a noble aspiration.