Mrs. Wright had been working on the quilt after she killed her husband. In her anguished state, she pretended that all was normal, refusing to think about what she had done or the consequences that might come. However, her stitching was not her usual perfection.
Mrs. Hale thought that the crazy stitiching might be used as evidence against her, to show that she had been mentally unstable at the time. It might be seen as evidence against a self-defense plea, should it come to that.
Whether or not the men would think to observe the stitching as evidence is another matter. But at this point, it was women vs. the men, and the troops were beginning to rally.