Mrs. Hale's action of mending the erratic stitching in the play Trifles can be perceived from a number of angles.

One potential reason is that Mrs. Hale is trying to conceal the way that the stitching looks because it gives away the state of mind of Minnie Wright. If the men see the level of anxiety that Minnie apparently lived under, they would have all the more reason to accuse her.

MRS HALE I'll just finish up this end. [Suddenly stopping and leaning forward.]
Mrs. Peters?

MRS. PETERS: Yes, Mrs. Hale?
MRS. HALE: What do you suppose she was so nervous about

Another reason may be that the stitching represents the one part in the country wife's life that lets her shine. The stitching requires talent and creativity; these are luxuries that can seldom be performed particularly in the busy duties that come with tending farms. It is possible that Mrs. Hale simply wanted to keep Minnie Wright's only source of feminine ability looking the way that it is intended to look; this would symbolically retain some dignity for a woman that has been accused of a horrible crime.

[Mildly.]
Just pulling out a stitch or two that's not sewed very good. [Threading a needle.]
Bad sewing always made me fidgety.

Out of these two potential reasons, it all points out that Mrs. Hale is trying to conceal from the men the reality of Minnie Foster; these are not the days when women would be given the choice of claiming temporary insanity, nor battered woman syndrome which are the precise two things that Minnie was suffering from. In a rural, old-fashioned county, Minnie would be tried at face value no matter if she had a perfectly explainable reason to do what she did to her husband. Hence, by concealing any possible piece of evidence, Mrs. Hale would have spared Minnie another "giveaway" of her state of mind. The evidence comes with the way in which Mrs. Peters responds to what Mrs. Hale is doing

MRS. PETERS: [Nervously.]
I don't think we ought to touch things.

Mrs. Peters understands perfectly the reason behind the mending but, as a woman, she puts aside the fact that she is the sherriff's wife and, instead, she takes the side of Minnie because she understands that the woman had been, indeed, the victim of cruel and ongoing spousal abuse.