Indeed, it is the men in this drama who are trifling [to use the dialectal meaning of worthless]. For, they are the ones who miss the critical evidence which indicates a motive for the strangulation of Mr. Wright. Ironically, the sheriff dismisses the importance of the kitchen, responding to the attorney's query if there is anything that would point to a motive, declaring that there is "nothing here but kitchen things."
However, as the women wait on their husbands, Mrs. Hale, wife of the sheriff, remarks,
"I don't know as there's anything so strange, our takin' up our time with little things while we're waiting for them to get the evidence....I don't see as it's anything to laugh about."
And, it is they who then discover the little canary whose cage door has been bent and its neck rung--the very evidence needed to point to a motive.
Your question asks who are the true trifles, so the answer to that can only be the men. They overlook things both because they don't understand or give credence to the ways of women but also because the physical trifles (as mentioned above) are so out of their realm of experience. They're supposed to be investigators who don't overlook clues--especially obvious ones--but the entire incident and the motivations for it are, frankly, beyond their realm of experience.
Woah - bit of a harsh response, isn´t it?! Whilst I agree with amy-lepore here I think the trifles in the play are the clues that the men overlook. The trifles for example regarding the quilt, the jams and loaf of bread that the women are able to use to piece together the picture of the crime and also solve it. The men walk through this female world ignorantly, underestimating the knoweldge and input that the women have.
The men, of course. They look down on the women's lives and the menial tasks the women perform. They do not acknowledge the depth of knowledge and/or skill needed to perform these tasks adequately. They simply act as if anything which does not require brute strength and force is not worth the time it takes to complete the task. How wrong and trivial they are.