The mood of "Miss Brill" is fairly whimsical and imaginative in the beginning. The first line of the story and the initial description of the setting is quite pretty and cheerful, introducing this mood, with "the blue sky powdered with gold and great spots of light like white wine splashed over the Jardins Publiques." The connotation of words like powdered and gold are quite positive, and the simile which compares the light to splashes of white wine feels both fun and sophisticated. Further, Miss Brill's own jaunty mood, especially as regards her little fox fur, which she refers to affectionately as a "Little rogue!" confirms the whimsy of the mood as well.
We do get small clues throughout the story that not all is as it seems to Miss Brill, though, so the mood change isn't as abrupt as it might seem at first glance. Miss Brill's fantasy that "They were all on a stage" is imaginative, but the strength with which she seems to believe and commit to the idea should strike us as a bit strange. Moreover, her plans to tell the man she reads to that she is "'an actress'" and has been "'for a long time'" make her seem rather more delusional than whimsical. In addition, the strange sentence, "No doubt somebody would have noticed if she hadn't been there," begins to point to Miss Brill's loneliness and sense of emptiness in her life, feelings of which she seems to be in denial until the end.