What is the poem "Ozymandias" mostly about?
This is a great question. Ostensibly, this poem, as the title indicates, it about a statue of Ozymandias, or Ramses II of Egypt, now "half sunk" in the desert sands. Whether the poem is really about this statue, however, is a different issue. While the poem describes the statue piecemeal—its "shattered visage" and its "sneer of cold command," now lost in a sea of desert—we can argue that it is really about the hubris of Ozymandias and the ultimately transitory nature of human power and human existence.
Ozymandias was once "King of Kings," proud enough to believe that people for years to come would look upon his works and "despair." The skill of his sculptor, certainly, survives to a certain extent: it is possible to read the personality of the king in the proud look upon his carved face. Now, however, what was once a colossal monument is now a mere "Wreck," beyond which stretches no great works but only an expanse of sand. Far from being a monument to Ozymandias's greatness, then, the remains of his statue are instead a monument to his arrogance in believing that he could have power even over death and time. His words take on a new meaning—the "mighty" will indeed "despair" upon seeing what has happened to this once great ruler, but not in awe of his works, as Ozymandias once hoped.