These are two very good texts to compare. Quite clearly you will want to compare and contrast the characters of Miss Brill and Minnie Wright and the way that both lead lonely, isolated lives as shown through the text.
Of course, one of the central aspects that you can contrast is the manner of presentation of this theme. "Miss Brill" by Katherine Mansfield is noted for its use of a stream of consciousness narrative that presents us with the thoughts of Miss Brill as they pop into her head. What is fascinating about this narrative is the way that the author gives us a complete character sketch of Miss Brill, even though she only really talks about the different people that she sees and hears on her trip out. Yet the way she reports this is enough to give us a real sense of who she is as a character and her terrible loneliness.
In Trifles, however, we never actually meet Minnie Wright. We are only given details about her life from the conversation of Mrs. Hale and Mrs. Peters, who tell us about her life before she was married and then how she changed so radically afterwards thanks to her husband, John Wright. Thus there are two very different manners of revealing the theme that you are focusing on that you would do well to consider in your essay.
If I were writing this assignment, I would select the following thesis statement:
Both "Miss Brill" by Katherine Mansfield and Trifles by Susan Glaspell depict the profound isolation and loneliness suffered by women, but in very different ways.
This thesis statement would give you scope to talk about the theme you have identified, but also you would have freedom to talk about the differing presentation of this theme. Good luck!