No joke. It’s like the old saying: You catch more flies with honey than you do with vinegar.
I can totally see Dark pretending to be as sweet as Mark but, where Mark is just naturally honest and caring, Dark would appear sweet and kind to manipulate more people into trusting Him.
Like in the “Chocolate” ending of ADWM, where Dark pretends to be concerned about the audience’s state of mind long enough for us to drop our guard. Only, He’d be like that ALL THE TIME.