I'd say that this poem's use of capitalizations is largely a matter of stylistic emphasis on Shelley's part, which (in my own experience at least) leads the reader to place added stress upon those particular words which Shelley draws the attention towards. Thus, the message in question reads something like follows (note, I've italicized these words for emphasis): "I am Ozymandias, King of Kings;/ Look upon my Works, ye Mighty, and despair." If you notice, the eye seems to be drawn towards certain words, and these words themselves each express something critical concerning the personality and intentions of Ozymandias himself, who is addressing future rulers, boasting of his own accomplishments, which he thinks will last for perpetuity. These capitalized words represent the core of what this message seeks to convey (a message which, in turn, is of critical importance to the larger poem it is a part of).
In his poem "Ozymandias," Shelley uses capitalization to point out one main thing, and that is that egotism and pride have short shelf lives; they don't last very long, and no matter how grand or powerful you were in your lifetime, eventually, everyone forgets and your power wanes. If you take note of the words you listed above, and how they are capitalized, Shelley is emphasizing how Ozymandias thought himself quite grand and royal at one point. He was not just a king, he was a King of Kings! He did not just accomplish many works in his lifeteime, he accomplished Works of great significance and importance. He challenges not only the mighty to worship him, but the Mighty; he is greater even than the mightiest of the Mighty. Adding capitalization to these words is a way to show how the ruler had grandiose assumptions about himself. He capitalizes his own titles, giving himself supreme importance. One has to assume that he instructed the inscription to be placed on the sculpture, so his selective capitalization reveals his own high opinion of himself.
Then, Shelley throws in the curve ball when he capitalizes Wreck. All of the greatness, the Mightiness, the Kingliness of this man, is now a Wreck. Not just a tiny wreck, not just a minor wreck, but a capital Wreck. Just as the ruler considered himself mighty in life, so is his demise, the Wreck that is now the ruins of his domain, and the nothingness that remains of his kingdom. I hope that those thoughts help; good luck!