Overall, the poem assumes a mocking tone. Percy Bysshe Shelley employs a number of techniques to emphasize the futility of man's desire to achieve immortality and he criticizes the arrogance and vanity of specifically those in power to assert their dominance and demand praise.
Firstly, Shelley indicates that the speaker's knowledge of the statue is not derived from personal contact with it, but is merely an anecdote shared by the speaker from information divulged by another who had actually viewed the statue. This in itself demeans the importance and value of the object, since the perception thereof is not based on a first-hand account but derived from the experience of a third party. As we all know, second-hand accounts are not very reliable and we mostly doubt their veracity. The irony is obvious: the one (Ozymandias) who had had his image chiseled in stone wished for it to be admired and appreciated by the viewer - in this instance the speaker is not the one who actually saw the statue - his perception of it is thus in doubt.
Secondly, the artist who sculpted the statue is given more praise for his great skill than the praise given to the image (or the one whom the image portrays). The sculptor 'well those passions read', whilst the image displayed a 'frown', 'wrinkled lip', 'and sneer of cold command'. This indicates a clear contrast. The diction conveys a negativity toward the one sculpted and is positive about the one who sculpted.
Thirdly, Shelley indicates through imagery that the 'vast' statue has become almost nothing and has thus lost its importance, impact and value. The statue is 'trunkless', 'half-sunk', 'shatter'd'. It is a 'wreck' and has suffered 'decay'. The repeated use of such negative images emphasizes the idea that Ozymandias' desire to achieve everlasting fame, glory and admiration was foolish. This is further supported by the alliteration, 'boundless and bare' and 'lone and level'. The statue is surrounded by emptiness and nothingness, which add to its meaninglessness.
Finally, it is not only the speaker who mocks Ozymandias' pointless exercise in vanity, but also the sculptor: 'the hand that mocked them'. The sculptor has, through his creation, ridiculed Ozymandias by the manner in which he transferred his image to stone.