In order to determine the tone of an author, it is often helpful to know something about the author's personal attitudes. That Kate Chopin wrote of women's repressed roles in the Victorian Age and that the theme of "self-assertion" is prevalent in many of her works is apparent. So, the attitude of this author in her story about Mrs. Mallard who feels "monstrous joy" at the tragic news of her husband's death reaffirms this attitude.
After Mrs. Mallard is given the news of her husband's death, she weeps "with sudden, wild abandonment" and retreats alone to her bedroom upstairs. There, she sinks into a roomy armchair:
She could see...the tops of trees that were all aquiver with the new pring life. The delicious breath of rain was in the air....The notes of a distant song which someone was singing reached her faintly....
From this description the reader senses a tone of jubliance as Mrs. Mallard becomes aware of sounds and sights that before were overshadowed by her repressed state.
From this point on, the tone of the story seems positive as Mrs. Mallard's "blood warmed and relaxed." She realizes that she will "live for herself" in this "moment of illumination." In fact, it seems almost as if Kate Chopin is cheering as the omniscient narrator who describes Mrs. Mallard as she "drinks in the elixir of life through that open window."
I would like to emphasize in the previous answer's use of the following citation as the embodiment of the state of mind of the protagonist
She could see...the tops of trees that were all aquiver with the new spring life. The delicious breath of rain was in the air....The notes of a distant song which someone was singing reached her faintly....
The narrator's attitude is very evident on that quotation, because it brings out the "coming out" perspective of a repressed woman finally brought out. The line "the notes of a distant song which someone was singing" basically denote an irony- the protagonist knew exactly what the problem was- she had known it all along, but continued depriving herself from her needs, and carried on with a life that was less than satisfying.
If one of my students were to write an essay on this question I would gladly accept the words: Ironic, sarcasm, deprived, repressed, and nostalgic as describing words that explain the behavior of the protagonist. For she felt all these emotions in a period of an hour, in addition to joy, freedom, liberation, self-love, independence, and serenity- All which then shattered down, but still left a stronger presence it its short-lived existence.