Shelley's sonnet 'Ozymandias' is an example of what is called the "ubi sunt" (Latin: "where are they") theme in literature, that deals with the fallen heroes of the past.
The greatness of Ozymandias is belied by the fact that the statue is fallen and partly destroyed. This suggests "sic transit gloria mundi" (so pass the glories of the world) -- that even the power of the greatest kings (as those of Mesopotamia or Egypt) is transitory.
Another theme is that Shelley's own poem (as Shakespeare's "Not marble ..) will outlast the works made of mere marble or bronze.
the significance of the inscription found on the base of the pedestal in Shelley's "Ozymandias" is to emphasise that he was the most powerful king that is he was the kings of kings