Sonnet
The hand that mocked
Tricky grammar: 'mocked' means both 'imitated' and 'ridiculed.' The sculptor's hand copied the king's sneer, but also outlasted him—the artist wins.
Ozymandias
Greek name for Ramesses II, who ruled Egypt 1279-1213 BCE and built more monuments than any other pharaoh. Shelley's using the most monument-obsessed ruler in history.
ye Mighty
The irony pivot: Ozymandias meant 'despair at my greatness,' but now it reads 'despair because even I became sand.' Same words, opposite meaning.