Okay, "Mack the Knife" is a lot less happy and bouncy as the original Moritatensong, which introduces Macheath/Mackie Messer as a generally unpleasant character, at the start of The Threepenny Opera.
But the really wild story is; Threepenny Opera was originally going to open with the beggar king's song. The actor playing Macheath wanted it to start with a song about *his*... cravat, of all things. Starting the opera with a song about Macheath's *character* is actually a compromise.