Drama, Mystery, Parable
It's no big secret how Trifles qualifies as drama. It's a play, right? It's a piece of literature written mostly in dialogue that's never fully realized until it's brought to life by actors in front of an audience.
This play also happens to be a mystery. Sure, it's pretty obvious from the beginning that Mrs. Wright killed her husband. Her excuse that she somehow slept through him being strangled beside her in bed is pretty flimsy. Unlike a lot of mysteries, though, this isn't really a whodunit. It's a whydunit. The main thing we want to know is why Mrs. Wright snapped and offed her hubby.
Trifles might also qualify as a parable since it's a pretty simple story with a strong message. Throughout, Glaspell whacks us in the face with just how disrespectful and condescending men can be towards women. Seriously, these guys are all-out jerks. Nowhere is this clearer than when the whydunit is solved, and we learn that Mrs. Wright snapped after her husband snapped the neck of her canary—and this was after years of neglect.
Yup, seems like Mr. Wright was really Mr. Wrong.