Is Mayella Ewell in Lee's To Kill a Mockingbird a static or dynamic character?

Mayella is a static character, because she is a supporting character and does not change. 

In literature, a static character is one who does not change from the beginning to the end of the book.  A dynamic character grows and changes.  This is why main characters like Scout tend to be dynamic, and supporting characters are usually static.  Mayella Ewell is a supporting character who does not change.

Mayella is not a character who appears directly much in the novel, even though she is very important to the plot of the book.  She is the one who kissed Tom Robinson and caused him to be accused of rape, but Mayella is not really an actor.  She gets pulled along with events. She is not dynamic, because she does not change from the beginning to the end of the story. 

We know very little about Mayella until the trial.  The first thing we learn about her is that she keeps geraniums on her otherwise trash-infested property.  At the trial, Scout feels sorry for her when she learns how miserable she is.  Mayella’s father’s testimony is coarse and uncouth, making jokes about her parentage and describing her rape in a brutal and crude manner. 

During Mayella’s testimony and Atticus’s cross-examination of Mayella, it becomes apparent that the poor girl has a miserable existence.  Her life is all about taking care of her many siblings and trying to feed them on the game her father shoots, since none of the welfare checks seem to make it into the house. 

As Tom Robinson gave his testimony, it came to me that Mayella Ewell must have been the loneliest person in the world. She was even lonelier than Boo Radley, who had not been out of the house in twenty-five years. When Atticus asked had she any friends, she seemed not to know what he meant, then she thought he was making fun of her. (Ch. 19) 

Tom Robinson testifies that Mayella was lonely, and he felt sorry for her.  It was a big mistake for him to say that, because the jury did not approve, but it shows that she really was quite lonely. Mayella had no one.  She couldn’t have a relationship with Tom Robinson because it would be considered inappropriate since he was black.  Although Mayella's lonely and sad life moves the plot of the story forward through the trial, this is the only place she appears.