The title of the play is oozing with irony. The title comes from this gem of a line from Hale: "Well, women are used to worrying about trifles" (132). He says this in response to the fact that Mrs. Wright seems to be more worried about her preserves bursting than she is about the fact that she's being held for murder.
After this line, all the guys yuck it up about how women never worry about important stuff, and then the guys head off to go do important man stuff like looking through the bedroom for evidence.
The title's irony rears its head when Mrs. Hale's and Mrs. Peters' concern with trifling women stuff ends up solving the mystery. Mrs. Hale and Mrs. Peters would never have figured out what happened if they hadn't stayed in the kitchen (a.k.a. the woman's place) and been looking through Mrs. Wright's quilt scraps. So it seems like all this unimportant woman stuff weren't just trifles after all. The title also works on a larger level: it also represents how women as a whole are treated like unimportant trifles.