Mayella's a Ewell, and everyone knows what the Ewells are like: ugly, shiftless, and trashy—they even live by a dump. But when she takes the stand, she represents something else entirely: a flower of "Southern womanhood," an idea that itself is, according to Atticus, a "polite fiction" (15.39). But to justify sending an innocent man to death, the jury has to believe in her as a representative of "fragile" white women everyone:
A young girl walked to the witness stand. As she raised her hand and swore that the evidence she gave would be the truth, the whole truth, and nothing but the truth so help her God, she seemed somehow fragile-looking, but when she sat facing us in the witness chair she became what she was, a thick-bodied girl accustomed to strenuous labor. (18.2)
In order to convict Tom, the jury has to believe in, or at least pretend to believe in, the fragile, helpless girl who gets taken advantage of by Tom, rather than see her as a desperate, lonely teenager who actively desires him. It's not just ideals of women at stake, but also of men:
"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 gentlemen don't wanta do nothin' about it then you're all yellow stinkin' cowards, stinkin' cowards, the lot of you. Your fancy airs don't come to nothin'—your ma'amin' and Miss Mayellerin' don't come to nothin', Mr. Finch-" Then she burst into real tears.(18.167)
Mayella's comment suggests that for men to be big brave heroes, they have to believe that women are helpless timid victims in need of protection or avenging. According to this logic, proper men have to take Mayella's word over Tom's, or risk having their Man Licenses revoked, because Man has been defined as He Who Protects Women, not as He Who Listens Carefully To All The Evidence And Makes A Rational, Considered Judgment Based On The Facts.
However we could argue this does