What makes the scene where all the lovers confront one another comic instead of tragic?

The scene is comedic because of the incredulity of the entire scenario.  The characters question whether they have dreamed the entire thing because the events of the night are so unbelievable and surreal.  The chances of Lysander falling in love with Helena, for example, are slim-to-none!  All four lovers have done things, also, that are uncharacteristic of themselves, so this adds to the comedy of this scene.