Mayella Ewell does not reveal her real feelings about Tom Robinson in Chapter 18 of To Kill a Mockingbird. Instead, she acts as a mere mouthpiece of her father, who caught her kissing Tom.
In his summation at the end of Robinson's trial in Chapter 20, Atticus alludes to Mayella, saying that she has broken "a rigid and time-honored code of our society," a tradition that if broken, a person is ostracized from society. She has broken this forbidden code because her desires have been too strong for her to resist. So, now in court she makes the effort "to destroy the evidence of her offence," as Atticus says, by accusing Tom of rape and pretending that she has done nothing wrong.
Coerced by her father to accuse Tom, Mayella states that she called Tom to break down a chiffarobe for her and earn a nickel. According to Mayella, she went into the house to get the nickel when Tom "run up" behind her, putting his hands around her neck and
"...cussin' me and sayin' dirt--I fought'n'hollered, but he had me round the neck. He hit me agin an' agin--...he chunked me on the floor an' choked me'n took advantage of me." (Ch. 18)
After Mayella gives this testimony, in her narration Scout remarks that like her father, Mayella feels confidence afterwards, but "there was something stealthy about hers, like a steady-eyed cat with a twitchy tail." Under the coercion of her father who witnessed her forbidden behavior, Mayella now places damning blame upon the innocent Tom Robinson. In order to not be accused of breaking a strong social code in Maycomb in trying to sexually entice a black man, she lies about what happened in Chapter 18.