Ozymandias is what the Greeks (namely, Herodotos) called him; it's a corruption of his throne name. It is not appropriate for common discourse re: Ramesses II and would probably get a lot of people confused with the Shelley poem.What's the difference between Ozymandias and Ramesses? Which is more appropriate?
In a marvel of consistency, people do, however, use the Greek-corrupted names of Iranian Great Kings like Xsayarsa and Darayavahush.