Casey's appearance in court is important to the outcome at trial. It's a matter of perception, and not just in our society or modern society, but as human beings. Sensory perception, particularly visual perception, is a major factor in our decision making process. Heck, even a female bird when choosing a mate will judge a male by his feathers.
Everything that a juror sees and hears in the courtroom will affect his or her decision. The writers of the United States Constitution understood this. That is why an accused is granted the right to appear in civilian clothing as opposed to prison garb, lest the presumption of innocence be impaired by the jury's perception of the defendant as a criminal. In our media age, the argument for allowing defendants to appear in civilian clothing at pretrial hearings is based upon the risk of prejudicing potential jurors.
The visual evidence in this trial will be horrific, to be sure, but no more than in other capital trials where the defendant was acquitted. In some instances, gruesome evidence presented at trial backfires for the prosecution because of the emotional impact on the jury and their perception of what they see before them. Most jurors have never seen skeletal remains, decomposed bodies, and severed limbs. When faced with the evidence they reach into their memories of past experiences for a frame of reference, but their is none. Or what they find is a connection to a movie or book they read where the villian was a dark, frightening character. Certainly not an attractive young woman. Granted, modern jurors are jaded, but not so much as we might think.
At any rate, this is far from a slam dunk case for the prosecution or the defense. Each side will have to play to the jury. For the defense, that requires utilizing every means to broaden the disconnect between the evidence and the accused. In that regard, her physical appearance and demeanor will be of vital importance.