A cemetery doesn't seem like the best place for a joke, but these dead folks beg to differ! Many of these individuals wanted their loved ones to be able to look down on their tombstone in remembrance in a happy light instead of a sad one. It's better to leave them laughing than crying.
This person left their loved ones with a grim reminder... We're all going to end up where they are at some point! They must have had a pretty dark sense of humor.
Making your gravestone an Ouija board is a total boss move. This person is literally asking for someone to try and communicate with them on their actual grave. I'm sure plenty of people have taken them up on that.