Could you please give me some insight on why Puck says "Lord, what fools these mortals be!" in act 3 in the woods?
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.
Puck is talking about everyone. Love makes fools of us all, acting in ways that we would never consider doing when not under its "spell." Puck, immune from the chaos he inflicts, watches in bemused fascination as the lovers run about and generally go to pieces all over the desire to love and be loved.
Puck is an ever-happy sprite who divinely intervenes, yet also observes from afar. He himself makes observations of all of the complicated love stories and mayhem around him. His line is not said in disgust, but in gladness that he can be observe from afar and not have the "drama" that the others have. "Lord What Fools These Mortals Be."