The message or theme of the poem (sonnet) is that man is insignificant and his efforts are vein when compared to the forces of time and nature. The subject matter, diction, figures of speech, sound devices and structure used by shelly convey the idea of permanence and impermanence.
To consider the issue of the power of time and nature the poet has the narrator reporting on a meeting with a traveler from Egypt who told of seeing in the desert the remains of a vast statue. Only the legs remained standing. The trunk was missing and the shattered face lay half buried in the sand, he told that the sculptor had skillfully captured the frown, the wrinkled lip, and sneer on the face ("well those passions read") clearly the forces of time and nature have lowered this once imposing statue.
Shelly praises the artist for his skill in understanding the nature of the king Ozymandias and representing the king's passions objectively and without distortion.
("Mocked" means both "made a likeness of" and "ridiculed". He had ridiculed Ozymandias because he had copied the king's passions in impermanent material and because he knew that material things come and go and Ozymandias would not be great for all time.) He had also recognized the cold, proud nature that made Ozymandias behave in the way he did ("heart that fed")
The poet further uses his subject matter to deal with the impermanence of all things in the face of time and nature when he extends his description of Ozymandias by stating in the sestet the words on the pedestal.
These words, "My name is Ozymandias, king of kings
Look on my works ye mighty, and despair!" are juxtaposed with the description of the desert sands in the last 3 lines,