I think that one way the poem contrasts with Neoclassicism is its view of political autonomy and positive freedom. Neoclassicist thinkers believed in wise rulers and if rulers reigned with such wisdom and rationality, there is little to stop their tenure being associated with longevity and historical validity. The poem's vision of a statue that time has desecrated and withered, with the notion of "King of Kings" ringing hollow after time brings to light the idea that all human accomplishments are victim to the passage of time. This repudiates the stress on human greatness and positive freedom brought out by the Neoclassical thinkers.
Neoclassicism is the movement in Europe in the 19th and 19th centuries in which the arts revived Greek and Roman influences. eNotes points out:
Ideally—and neoclassicism is essentially an art of an ideal—an artist, well schooled and comfortably familiar with the canon, does not repeat it in lifeless reproductions, but synthesizes the tradition anew in each work."Ozymandias" is about the ruins of a statue of a leader from long ago. The statue is in many pieces and all that remains of this leader's great kingdom is this statue. The rest has been engulfed by time and the desert. During the Romantic Era, when Shelley wrote, the French Revolution was a great influence on its writers. Many protested the growth of the monarchy's power in Britain and Shelley was no different. In contrast to the ideal of the great power of the Greek and Roman empires, this poem clearly shows how despite having great power, eventually it will collapse and time will remember little of it.