In the poem, the words on the pedestal read,
Look on my Works, ye Mighty, and despair!
These words certainly contrast with what we are told of Ozymandias's heart. Indeed, these words suggest a cruel, tyrannical character. He is, after all, ordering people to "despair," or lose hope, when confronted with the awesome scale of his power. Yet, earlier in the poem, we are told that he had a "heart that fed." He cared for his people and made sure they had enough to eat. In this sense, he comes across as a protective paternal figure, in contrast to the cruel tyrant implied by the words on the pedestal.
It's also possible to argue that the words on the pedestal of the statue contrast with what we know of Ozymandias's actual deeds. From the poem, the only accomplishment, or deed, of Ozymandias's that we know about is the statue itself, which now lies crumbled, shattered, and forgotten in the "lone and level sands." This hardly seems like a deed worthy of the demand to "Look on my Works, ye Mighty, and despair!" However, if we have some knowledge from outside of the poem of Ozymandias's deeds, then the words on the pedestal perhaps seem appropriate. Ozymandias is the Greek name for Rameses II. Rameses II was an Egyptian pharaoh (1303–1213 BCE) known also as "Rameses the Great." He led many successful military campaigns, oversaw the construction of cities, and reigned for sixty-six years, from 1279 to 1213 BCE. In this sense, then, he really does seem to have been as powerful as he claims in the words inscribed on the pedestal of his statue.