The irony about Bob Ewell's response to Mr. Gilmer's question about being ambidextrous is that he answers the query completely backwards because he does not understand the meaning of the word. To be ambidextrous means that a person can use both hands equally well; an ambidextrous person, for example, might be able to write or eat as easily with his right hand as he can with his left. When Mr. Gilmer asks Bob Ewell if he is ambidextrous, Mr. Ewell answers definitively, "I most positively am not, I can use one hand good as the other". Mr. Ewell should have answered that he is indeed ambidextrous, precisely because he can "use one hand good as the other". As it is, he says that he is "positively not ambidextrous", emphasizing his point by repeating his incorrect definition of the term, "one hand good as the other", twice (Chapter 17).
The irony strikes the reader as situational (versus dramatic or verbal/sarcasm). A reader expects that an adult man would know the meaning of "ambidexterous;" and when Ewell does not, the reader is surprised. In other words, a reader expects that a situation will move in one direction when ironically it moves in an opposite one. In this excerpt the topic of satire Lee aims to introduce is the educational system (one of her top societal criticisms). The message she sends then to both the intended and the real reading culture could be composed in any number of ways. "The educational system is/was not effectively preparing individuals for the most basic challenges in life." "Every individual should have access to an education that at the very least allows him or her to both read and to write." Et cetera, so forth, and so on.... ; )