Concerning the play, Trifles, and its morality or lack of it, it's been said that the only immoral art is bad art. Whether or not one agrees with that, morality is not what the play is essentially about. It's a feminist story demonstrating the plight of women in a patriarchal society, in a society dominated by men.
The husband is immoral, if you want to think in those terms, and the other men are sexist and condescending. To think about the morality of the wife or the other women is a bit beside the point, however; it's something you can think about for the sake of creating a discussion, or for the purpose of discussing ethics, but it's not an essential element of the story. A woman is isolated and abused by her husband, and she fights back the only way she can. To say the story is immoral is to apply your own views and values to the work of art: again, it might make for a good discussion and might serve as a great prompt in an ethics class, but it's beside the point as far as the work of art itself. A reader's job is to understand what the work of art reveals about human existence, not to morally denounce the characters.
On the other hand, one might say the work is too didactic, designed primarily to prove a point, much like an argument essay. Some would say if the work is primarliy didactic, then, as a work of art, it falls into the category of propaganda. If you agree with this then, perhaps, you might say it's bad art. And if that's the case, then you might say that since it's bad art, it is immoral, according to my opening statement.
In the terms you're asking about, though, no, Trifles is not immoral.