I agree with the part about the "cat with a twitchy tail," but Mayella is really not intelligent enough to be considered "stealthy" or "steady-eyed." This description by Scout comes during her initial testimony given to the prosecutor, and
... Mayella's recital had given her confidence...
But Mayella's mettle is soon shattered by Atticus's appearance--she begins crying almost instantly during his cross-examination--and she soon shows herself for what she really is: an illiterate, unsophisticated "thick-bodied girl accustomed to strenuous labor." Atticus confuses her with nearly every question, and she soon contradicts herself about the events that she claims to have taken place. Jem and Scout both wonder if "she got good sense," and Judge Taylor even asks
"What's the matter with you?"
Although Mayella remains firm at the end of her testimony when she refuses to answer any more questions, she reveals herself as a friendless, lonely girl, so desperate for attention and companionship that she is willing to invite Tom--telling him "come here, nigger"--into the Ewell house so she can corral him with hugs and kisses. Scout even seems to have changed her mind.
I guess if she hadn't been so poor and ignorant Judge Taylor would have put her under the jail for the contempt she had shown everyone in the courtroom.