Puck's statement is somewhat ironic, for if ever there were a fool, it would be Puck here. He has just royally messed up the task Oberon set for him and made all the wrong people fall in love. Oberon has just fixed things so that all of the right people are supposed to fall in love. Puck is a trickster. He's the one who plays with people in order to get them to do stupid things so he can laugh at their expense. Not one to fall in love himself, he is utterly enjoying the way the mortals are behaving. To him, the lovers' antics are like a comic event. When he says these lines to Oberon, he is saying he wants to watch:
Shall we their fond pageant see?
Lord, what fools these mortals be
Love makes mortals more foolish than any trick he could devise. To Puck, this is entertainment at its finest.