What does Scout comparison of Mayella to "a steady eyed cat with a twitchy tail" suggest?

Scout viewed Mayella's initial testimony as steady (calm, convincing). The "twitching tail" would lead us to believe that Mayella was confidant and sure of herself. Considering the lawyer for the prosecution was questioning her, she might very well have appeared sure of herself. None-the-less, that didn't last long. Atticus' questioning destroyed her composure almost immediately. 

This is foreshadowing Tom Robinson's fate. Mayella is described as a cat, and Tom Robinson is one of the 2 mockbirds in the novel. Cats are known to kill birds. In other words, this is foreshadowing how Mayella will be the death of Tom Robinson.