There are several characters in A Midsummer's Night Dream that undergo ironic changes. For example, Lysander is ironically transformed into loving Helena instead of Hermia. Likewise, Demetrius is ironically transformed into loving Helena, whom he shunned at the beginning of the play. However, Bottom is the most ironic character, not just because of the ironic change he undergoes, but because of who he is as a character.
One of the ways in which he is an ironic character is that he is very conceited about his abilities as a performer. He envisions himself being able to give a grand performance as Pyramus, 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, the reality is that, just like the other mechanicals, he is actually too uneducated and too unskilled to be able to give a convincing and winning performance. We see an example of his lack of education and skills when, in their rehearsal, we see him come up with ridiculous solutions for their stage set problems, such as having an actor play the part of the wall, and also when we see him botch up his lines, such as referring to Thisbe's breath as smelling like an "odious" flower (III.i.75).
It is due to his conceit and ridiculousness that inspires Puck to transform Bottom into the proverbial "ass," which brings us to the second way in which Bottom is the most ironic character. Bottom actually says his most humble an intelligent lines after having been transformed into a donkey. Bottom delivers his most intelligent lines after Titania proclaims she loves him. His response actually captures one of the play's most central themes, showing us just how irrational love is, when he says:
Methinks, mistress, you should have little reason for that. And yet, to say the truth, reason and love keep little company together now-a-days. (III.i.134-136)
Bottom also delivers his most humble line in response to Titania. When Titania declares that he is as wise as he is beautiful, Bottom denies it, saying, "Not so, neither," while previously he would have proclaimed his own merits (140).
The fact that Bottom is conceited about his own skills when in reality he has none, and the fact that he speaks his most humble and intelligent lines as a donkey, shows us that Bottom is indeed the most ironic character in the play.