I think Shelley feels that fame and power are meaningless - they don't add up to anything in the end since we all end up worm food. Just as Ozymandias' great monument deteriorated over time, so does a person's fame/power. This poem is an example of verbal irony in that the plaque on the bottom of Ozymandias's once great monument says, "Look on my works, ye mighty, and despair, however his entire monument has been eroded by nature and that even nature has power over the mightiest of men. Shelly is also making a satirical comment on the vanity and supposed glory of past rulers of Egypt. What the pharaoh once thought was great is now a barren wasteland. .
Ozymandias had the monument made to prove that he was great, but all that remains of Ozymandias's power has disappeared. All we are left to admire is the work of an unknown sculptor. So sculptors art (like a poets) outlives that of what he represents. There is another theme of the poem and that is Time. The emphasis is on the fact that Ozymandias wanted to be known for ever, but now he is forgotten. Shelly might also be mocking Ozymandias by writing about what happened to this phareos once great kingdom and his monument. Shelly could also be using Ozymandias to prove a point by showing us all that time and nature consume all, but instead of letting Ozymandias fade into nothingness Shelly writes about him thinking that his own work will not end up like Ozymandias kingdom thus letting Ozymandias live forever and actually letting Ozymandias get what he wanted in the end - eternal life.