In A Midsummer Night's Dream, Puck changes Bottom into the proverbial ass because that is precisely how Bottom is behaving. In other words, Bottom is behaving like an idiot. We especially see Bottom acting as an idiot when, in his excessive enthusiasm, he asks to have every lead role in the play. Quince has already asked him to play Pyramus, the lead male role, but when Flute hesitates to accept the female lead, Bottom says that he can play both parts. Likewise, when Snug hesitates to play the lion, Bottom says that he can play that part as well. Obviously, it is very ludicrous to think that Bottom could play all three parts at once. In addition, Bottom thinks a great deal of his skill as an actor, as we see in his lines, "If I do it, let the audience look to their eyes; I will move storms" (I.ii.22-23). However, we see just how ludicrous his overconfidence is in the rehearsal scene in which he keeps messing up his lines, such as the line, "Thisbe, the flowers of odious savors sweet--" (III.i.75). Obviously, "odious" is the wrong word as it refers to something that is "repugnant," or "disgusting" (Collins English Dictionary). Instead, Bottom was supposed to have referred to the flowers as having sweet "odors," meaning scents.
After Puck changes Bottom into a donkey, he is still an idiot, but he actually says something very intelligent; something that captures the heart of two of the play's themes. When Titania proclaims that she loves him, Bottom makes the very astute statement that there is no real reason for her to love him, and yet, reason and love don't really have anything to do with each other, as we see in his lines:
Methinks, mistress, you should have little reason for that. And yet, reason and love keep little company together now-a-days. (III.i.134-136)
This captures the heart of Shakespeare's themes that love is irrational and that the human mind is irrational as well.
Hence, we see that after Puck characterizes Bottom's foolishness by turning him into the proverbial ass, ironically, Bottom changes by actually making one very intelligent statement.