A "sneer" is usually deemed a disrespectful expression or attitude which is dismissive of others. When one has a "sneer" it means that one has a low opinion of or little care for the other at whom the sneer is directed. It is an expression of scorn or contempt. It evokes haughtiness or snobbery, normally associated with the very wealthy or privileged who believe that they are above the ordinary and are superior in every sense. It is disrespectful in the extreme and the purpose is to make the one to whom it is directed feel insignificant and belittled. The expression is born out of belligerence and an autocratic attitude.
The sculptor almost perfectly captured Ozymandias' sneer and conveyed his uncaring attitude toward whomever he came in contact with. More specifically, the sneer was directed at his followers - the citizens whom he commanded. He treated them with derision. To emphasize the "sneer," Shelley uses the descriptor "cold" which implies an uncaring, dismissive nature. Associated with "command," this means that Ozymandias had no care for his followers or their circumstances. He was selfish and sought only self-aggrandizement, clearly shown by his investing so much in having statues erected in his honor. He saw his followers as mere pawns, slaves there to do his bidding.
The repetition of the "c" further emphasizes how brutally cold and domineering Ozymandias was as a leader. He did not have his followers best interests at heart, but was completely uncaring and dismissive. The "c" is a harsh sound and conveys not only the harshness expressed by Ozymandias himself, but is also suggestive of the environment in which he ruled.
Ozymandias was so steeped in his own self importance that he even challenged others to cringe at the greatness of his works and be overcome with despair. Truly the signature of a remarkably inflated ego! However, it has all come to naught, for all that remains of his superciliousness is a broken statue in an empty, vast desert.
How the mighty have fallen!