In "Ozymandias," what does the expression on the face of the sculpture convey about the king's personality?
First, what we have is a poet's retelling of a traveler's account (actually Diodorus Siculus, an historian, not a traveler) of a sculptor's vision of Ozymandias, and thus we are only getting a sense of what layered narrators want to convey about the personality of Ramesses II; as the pharaoh in question died some 1000 years before Diodorus wrote his history and nearly 3,000 years before Shelley wrote the poem, we can only derive information about the character of the pharaoh as portrayed in the poem, not his actual historical character.
The poem describes the expression on the head of the sculpture as follows:
...a shattered visage lies, whose frown,
And wrinkled lip, and sneer of cold command...
The word "frown" makes the readers assume that the model had a harsh, perhaps angry, disposition as does the phrase "wrinkled lip" which evokes sneering. The words "cold command" suggest a powerful autocrat who is emotionally cold and lacking empathy.