Mayella Ewell is not a sympathetic character at all in the book. In other words, she does not do the right thing. She is selfish and self-preserving. From this perspective, we can say that she does not think of others enough - hence, not sympathetic. That said, this does not mean that we cannot pity her. She is in a very difficult situation.
Since Bob Ewell is her father, Mayella has very little. Also she has to take care of the children and she is lonely. So, when she tries to seduce Tom Robinson in her desperation, we are sympathetic. When her father walks in while she is trying to seduce Tom, and he beats her, we are even more sympathetic. More importantly, we can see why she does what she does - accuse Tom Robinson falsely. However, this does not excuse her from her guilt.
Atticus said it best:
“I have nothing but pity in my heart for the chief witness for the state, but my pity does not extend so far as to her putting a man’s life at stake, which she has done in an effort to get rid of her own guilt.
“I say guilt, gentlemen, because it was guilt that motivated her. She has committed no crime, she has merely broken a rigid and time-honored code of our society, a code so severe that whoever breaks it is hounded from our midst as unfit to live with. She is the victim of cruel poverty and ignorance, but I cannot pity her: she is white.