Marriage is seen as an controlling institution which denies women the ability to voice their own autonomy. This is seen in several instances in the short story. The first would be Louise's initial reaction to news of her husband's death. The narrator makes it fairly clear that Louise is supposed to mourn, as per social convention. Pay attention to the tone and language used to describe this. Even from the earliest stages of the narration, the idea is conveyed that while Mr. Mallard was not that bad of a guy as far as men goes, Louise is supposed to feel a certain way. When Louise goes upstairs and is alone, she experiences quite a different reaction for she is now able to accept her own autonomy, her own freedom, and her own voice. The narration swells as it enters her mind, indicating that marriage, the institution and practice of it, helped to keep these elements repressed or lulled into dormancy.
I think you would have to say that the narrator has a negative attitude towards the institution of marriage. At least, the narrator gives to Mrs. Mallard some very negative thoughts about the institution.
According to the narrator, Mrs. Mallard sees marriage as something of a prison. Even though she loves her husband, and even though he has never looked at her except with love (that is pretty amazing), she still is happy not to have to spend the rest of her life with him.
If these thoughts are consistent with the narrator's beliefs, then the narrator is very opposed to marriage as an institution.
Kate Chopin's narrator in her short story "The Story of an Hour" is an omniscient narrator that reveals and interprets Mrs. Mallard's thoughts. She reveals these thoughts, as is accepted and expected according to the conventions of fiction writing. You would want to think of this narrator as revealing, rather than giving, thoughts or attitudes.
As such, the narrator does not present an attitude toward marriage. She simply reveals the actions, dialogue, and thoughts that form the plot.
For instance, the narrator writes:
There was something coming to her and she was waiting for it, fearfully. What was it? She did not know; it was too subtle and elusive to name. But she felt it, creeping out of the sky, reaching toward her through the sounds, the scents, the color that filled the air.
The narrator here interprets the thoughts and feelings of Mrs. Mallard, but she is not expressing any attitude of her own. At most, one might say that the narrator is sympathetic toward Mrs. Mallard.