In my scholarly reading of literary criticism about To Kill a Mockingbird, I have never come across a definitive, well-agreed to comment on this.
The references to the chiffarobe occur in both Tom Robinson's testimony and Mayella Ewell's testimony. As Tom references this piece of furniture he reports that Mayella had an item on top of the chiffarobe and she asked him to get it. He has also been asked to bust up a chiffarobe in the past.
Mayella reports that the day in question was the day that he had intended to come bust up the chiffarobe.
Thus, on an interpretive level, I look at the chiffarobe in two ways. First, I see it as a symbol of confusion. The piece of furniture has a door like a closet, but drawers like a dresser. It is like it can't decide what it is. During the trial, Mayella struggled between keeping the truth and her rehearsed lies straight in her head. It is also in these moments with Tom that she is trying to assert an identity, other than the one she has.
Another way to look at it is that the chiffarobe is a doorway. This is a doorway that Mayella walks through when she is feeling ready to commit social crimes.