The poem actually presents more than one message.
Firstly, the poem emphasises the transient nature of existence. Time does not stand still and as it marches on, things change. All matter, irrespective of its nature, suffers from the ravages of time in some or other way. In the poem, this is illustrated by the change Ozymandias' colossal statue has undergone.
Two vast and trunkless legs of stone
Stand in the desert . . . Near them, on the sand,
Half sunk, a shattered visage lies,
Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.”
The two extracts clearly indicate that the statue has undergone a transformation for the worse. When Ozymandias had this monument built, he probably imagined that it would exist for all time. Its sheer size, he might have believed, would stand as a permanent testament of his power and achievement. Unfortunately for him, all that remains of his wondrous creation are the two massive legs of stone, a face and a pedestal - 'nothing beside remains.'
Added to this, the word 'decay' clearly indicates that the statue was unable to survive. The point is further emphasised by the very apt use of alliteration in 'boundless and bare' and 'lone and level sands.' The first reveals the vast emptiness of the desert, whilst the second serves as a metaphor for time, as in the phrase, 'the sands of time.'
Secondly, the poem mocks the foolish idealism of an arrogant and cold-hearted dictator. It is clear from the poem that Ozymandias was supercilious and uncaring as suggested by the lines:
And wrinkled lip, and sneer of cold command,
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things
Ozymandias presented a stern visage and he had a sneering look about him. He looked down on his subjects and ruled with a cold heart. The sculptor had skilfully etched these into the sculpture. Added to this, Ozymandias had the arrogance to believe that his power would create fear in all who were exposed to him. The legend on the pedestal signifies his great vanity:
‘My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!'
The irony is that Ozymandias did not seem to realise that his power was limited. He would pass on and so would his power. Death is the great equaliser, but he thought he would survive beyond that, and ironically, he did not. All that remains of his greatness is this broken effigy now being critically spoken of by the speaker. The speaker is not in awe, neither is he in 'despair' for Ozymandias might has ceased to exist. It is quite ironic that the speaker expresses greater admiration for the skill of the sculptor than for the sculpted subject.
Ozymandias must have surely turned in his grave at this.