The mechanicals are shown here outside of their context. In town they were silly enough at times, but here in the forest, they are truly exposed, and most exposed is, of course, Bottom. When Bottom warns against bring lions among ladies, or when he calls the lion a "wild-fowl," he is shown to be a wonderfully overblown fool. He's stuck in the literal, seeking an almanac for the moon to make sure the actual moon is right for a time of make believe. As a result, he (and they) are ripe to be made fun of when Puck appears, and it is only right that Bottom will soon be a literal ass.