The general tone of "The Triumph of Life" is despairing, if not completely despondent. Shelley presents us with a view of human history in which a succession of noble spirits has been crushed beneath the inexorable Chariot of Life. At the same time, he defiantly exalts those like him, the "sacred few" who have not compromised with the values and fleeting fancies of earthly life. Even in the gloomiest lines of the poem, there still remains a characteristic spark of the old Shelleyan élan and vigor, which hints at the possibility of a bright new beginning that transcends the temporal world and all its disappointments.
In the passage referred to in the question, the philosopher Rousseau has been seized by a new vision after drinking deeply from the cup of knowledge. Now he can see life more clearly, can see it for what it really is: a grotesque pageant of phantoms and shadows in which we dance insanely and unthinkingly in the wake of the Chariot of Life. Previously, Rousseau had been blinded by the light of his own philosophy in seeing nature as pristine and incorruptible. Yet after drinking of the cup of knowledge, Rousseau sees the truth: that it is the very natural life he previously extolled and romanticized that corrupts and triumphs over the human spirit.