I believe what you're referring to is when Mayella takes offense to Atticus's questioning at the trial because he calls her ma'am. During his questioning of her, Atticus repeatedly calls her "ma'am" and "Miss Mayella;" however, Mayella Ewell is almost certainly treated disrespectfully in her everyday life by her father, so she thinks that Atticus is making fun of her by using manners to speak to her. She abruptly screams out during the trial:
"I got somethin' to say an' then I ain't gonna say no more. That nigger yonder took advantage of me an' if you fine fancy gentleman don't wanta do nothing about it then you're all yellow stinkin' cowards, stinkin' cowards, the lot of you. Your fancy airs don't come to nothin' -- you're ma'amin' and Miss Mayellaerin' don't come to nothing', Mr. Finch." Then she burst into real tears. Her shoulders shook with angry sobs.
This is an important development, because it illustrates to the reader exactly how sad and small of a life poor Mayella has. It helps set up the next chapter, when the reader finds out that it was Mayella who tried to seduce Tom Robinson. By understanding how desperate she is for kindness, the reader can understand why she would break such a stringent rule in Maycomb by kissing a black man.