The mood or atmosphere of "The Yellow Wallpaper" is eerie and ominous, foreboding.
The woman is suffering from post-partum depression and kept prisoner in her room--she's not even allowed to see her baby. No one listens to her or takes her ailment seriously. She seems doomed from the start.
The mood is established by the woman herself--the first-person narrator--by her description of the setting. The isolated spot the couple venture to for her cure is "ancestral," maybe "haunted," at least in her imagination. There is "something queer" about it. It "came cheaply," and she speculates that there must be a reason for it. Her husband laughs at her, and she suspects that she is taking so long to get better because he is a physician.
All this combines to create the eerie, ominous, and foreboding mood.