There is no single overarching mood in To the Lighthouse. Perhaps one of the effects of Woolf’s narrative style that zooms into a moment at the highest level of detail—only to whiz back out as an entire decade passes—is that the mood changes with the thoughts of the characters being explored at any given moment.
The effect of all this motion between characters and time scales, however, is to suffuse the novel itself—or at least the omniscient narration—with an existentially anxious mood. After all, Woolf uses the story to explore some big and unsettling subjects, from the painful struggle of artistic creation to the meaning of life itself. Consider the middle section of the novel, "Time Passes," in which the narrator—flying between seasons a paragraph at a time—relates the joyous affair of Prue Ramsay’s marriage in Chapter 6:
The spring without a leaf to toss, bare and bright like a virgin fierce in her chastity, scornful in her purity, was laid out on fields wide-eyed and watchful and entirely careless of what was odne or thought by the beholders.
[Prue Ramsay, leaning on her father’s arm, was given in marriage that May. What people said, could have been more fitting? And, they added, how beautiful she looked!]
Just a paragraph later, the reader learns of Prue’s death:
As summer neared, as the evenings lengthened, there came to the wakeful, the hopeful, walking the beach, stirring the pool, imaginations of the strangest kind…. Moreover, softened and acquiescent, the spring with her bees humming and gnats dancing threw her cloak about her, veiled her eyes, averted her head, and among passing shadows and flights of small rain seemed to have taken upon her a knowledge of the sorrows of mankind.
[Prue Ramsay died that summer in some illness connected with childbirth, which was indeed a tragedy, people said. They said nobody deserved happiness more.]
In Woolf’s narration, even death is relegated to a parenthetical observation. But the existential bent of the novel, and the anxiety it may entail, do not create a mood of despair. Instead, To the Lighthouse becomes a meditation on the power of life and artistic creation even in the midst of the messiness of living and dying. It is ultimately optimistic.