In this poem, Shelley is saying that the power of kings is totally illusory. They think that they are great and powerful (as Ozymandias did) but they and all their deeds will, in the end, crumble into nothing. To me, this is the main theme of the poem.
Ozymandias was, in his day, a tremendously powerful king. He was able to cause people to create these great statues of him and he was arrogant enough to say that the "mighty" should despair when they saw how great he was.
But now what is left of his kingdom (in the poem)? Nothing. Just ruins of statues mostly buried in sand. All of the king's efforts and all of his power came to nothing.
"My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!"
From these lines, Shelly tells us that Ozymandias was very powerful during his times and feared.His power is also shown by the fact that even the 'mighty' despaired. However, as the poem progresses, we get to know that all that remains is a decayed and forgotten wreck.
"Round the decay
Of that colossal wreck"
From the above, Shelly points out the fact that everything is short lived and that nothing lasts forever; that the power of the kings last only till they live; that their kingdom and prominence will soon be forgotten.