What is the irony about Bob Ewell's response to Mr. Gilmer's question about being ambidextrous in To Kill a Mockingbird?
The irony attached to Bob Ewell's response to Mr. Gilmer's question about his being ambidextrous is that Ewell denies that he is "certainly not" ambidextrous, but adds, "I can use one hand good as the other," so he admits to it.
Of course, it is obvious that the uneducated and ignorant Ewell has no idea what the word ambidextrous means since he has inadvertently re-defined it when he asserts that he can use one hand as well as the other. In actuality, he thought that he was contradicting its meaning, not affirming it, by saying that he can use both his hands equally well.
As Scout listens to him on the witness stand, it becomes apparent to her just how backward and crude this man actually is as he uses vocabulary that is inappropriate to the courtroom. His descriptions, for instance, of what has occurred with Mayella is described in low terms, to say the least. At one point, for instance, Judge Taylor finds it necessary to tell Ewell to confine his testimony within the confines of Christian English Usage.