The inscription on the pedestal reads:

"My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!"

The words are a proclamation of Ozymandias' hubris. He saw his works as a reflection of his greatness and superiority and believed that they were incomparable. No one, no matter how great or glorified, would be able to match his self-proclaimed glory. Ozymandias claims that just viewing his works would drive even the greatest to despair for they would never be able to equal him.

The words are not a challenge to God, but to other leaders. "Mighty" is capitalised not because it is necessarily a reference to God, but to emphasize the greatness of other rulers who might also deem themselves incomparable. However, Ozymandias stresses through this inscription that he is by far the greatest, and whoever seeks to be his equal would be utterly distraught, for they would never succeed.

One may argue that Ozymandias' remark is also directed at God and that he might deem himself greater. In this sense then, he sees himself as a god. It is ironic in this context then, that the reference to "king of kings" is not capitalized, which would have been a definite indication that he also perceives himself as God's superior. 

It is ironic that this arrogant declaration is followed by the lines:

"Nothing beside remains. Round the decay
 Of that colossal wreck, boundless and bare
 The lone and level sands stretch far away."

Ozymandias' boastful vanity is only just that - a boast, for practically nothing of his greatness has survived: only a "colossal wreck" surrounded by "lone and level sands" which stretch into the distance.