There is really nothing in Trifles that can be considered comedic. The entire play is a suspenseful drama about the effects and consequences of spousal abuse; only the men find humor in the women's apparently inconsequential focus on "trifles." The events and realizations in the play are anything but comedic, though, and it is hard to laugh along with the men when it is obvious that they are gaining humor from misogyny.
However, the play can not really be deemed a tragedy either. The events that are revealed are come from the emotional abuse of Mr. Wright on his wife Minnie, and while that is itself tragic, her eventual response removes moves into revenge and actually resolves her emotional issues. The tragic events all occur long before the play itself; Minnie's creativity is stifled by her husband, and he emotionally abuses her to the point of killing her pet canary. Everything in the play itself is suspenseful, but not tragic; the outcome may be entirely unavoidable considering how badly Minnie was treated. The play therefore should not be classified as either comedy or tragedy, but instead suspense or straight drama (it could also be classified as a dramatic mystery).