A lot of the mood of "The Yellow Wallpaper" has to do with nervousness and madness. That unsettling, uncomfortable feeling is something the protagonist shares with the reader as both start to question what exactly is the truth about the wallpaper.
The woman who narrates the story is first brought to the room to heal, as she seems to have postpartum depression. For most of the story, it's unclear whether the narrator gets better or worse, and whether the room is supernatural or not. Her attitude regarding her baby is one of the only places where the narrator seems to comply with the initial diagnosis—she claims she cannot bear to be near the child.
From there, the story unfolds into confusion and many possible layers interpretation. The narrator's physician husband insists on a "rest cure," depriving the woman of any mental stimulation, company, and so on. She is not allowed to work or write; she's discouraged from socializing or exercising too much.
It is no wonder she starts to question his methods and her own sanity. As she sits day after day in that strange room, both the narrator and the reader get more anxious, which could be called the main mood of the story.
Is she losing her mind? Is the room supernatural? What happened in the room previously? (It's quite clear to the reader that there was no "boys' gymnasium" in that nursery, as the narrator assumes. The bars on the windows, the nailed-down bed, the torn wallpaper "just as far as she can reach from the bed"—they all point to someone being restrained there.)
"The Yellow Wallpaper" is one of the finest examples of the unreliable narrator, a technique that doesn't allow the reader to fully trust the narrator's judgment or objectivity. As she becomes more worried and uncertain about the happenings in the room, so does the reader.