Puck calling mortals fools is ironic because he is the one causing them to act foolish and because the fairies Titania and Oberon also act very foolish.
There are a couple of reasons that Pucks’s condemnation of mortals is ironic. First of all, he is the one causing a lot of the foolishness. The people he is anointing cannot control their behavior. It is a result of a magic spell. So Puck is laughing at people for acting just the way he is forcing them to act.
Then will two at once woo one;
That must needs be sport alone;
And those things do best please me
That befal preposterously. (Act 3, Scene 2)
However, the other aspect of the people being fools has nothing to do with magic. Love makes people do foolish things, as the play demonstrates. The irony is that the mortals are not the only ones acting foolish. Titania and Oberon certainly engage in their share of foolish behavior. They fight and make up, and they drag the whole forest into their mayhem.
Titania and Oberon fight over their jealousies. Oberon is jealous about the changeling Titania has. He is also jealous because he thinks she has something for Theseus and Titania is jealous because she thinks he has something for Hippolyta .
These are the forgeries of jealousy:
And never, since the middle summer's spring,
Met we on hill, in dale, forest or mead,
By paved fountain or by rushy brook,
Or in the beached margent of the sea,
To dance our ringlets to the whistling wind,
But with thy brawls thou hast disturb'd our sport. (Act 2, Scene 1)
Titania’s mood is not good for the forest. I would consider her behavior foolish, and Oberon’s too. They both kept pushing each other’s buttons, even though as fairy king and queen their behavior affected the entire forest.
In the end, foolishness abounds in this play for both mortals and fairies. Magic or not, people in love sometimes act in ways that make no sense. Jealousy can hit anyone, mortal or not.