In context, the inscription on the base of the ruined statue is addressed to other kings. Through the inscription, Ozymandias says:
My name is Ozymandias, king of kings: Look on my works, ye mighty, and despair!
The phrase "ye mighty" suggests that Ozymandias means for future and present kings to survey the enormous building projects he has constructed and "despair" of ever being as powerful as he is. So he is essentially telling other monarchs that they will never achieve the level of greatness that he has. Of course, the supreme irony of the inscription is that it is at the base of a ruined statue, and that the "works" that Ozymandias intends people to be humbled by are in fact ruins as well, covered by drifting sands. So the inscription, rather than drawing attention to the power of the great king, actually underscores his humanity and his mortality.