How are the words on the pedestal of ozymandia's statue ironic?

On the pedestal of his statue the words, "My name is ozymandias, the king of kings. Look upon my words ye mighty and despair" is written. These words are ironic as the statue of the king who considered himself as the "king of the kings"

i. e the mightiest of all is now in a very miserable condition. The statue is trunkless and half sunken in the sand showing no signs of his mightiness.