In the poem, Shelley states a couple of things about the statue that suggest that Ozymandias was a despot. He points to the "sneer of cold command" and to the inscription on the pedestal. Both of these imply he was a despot.
If you are asking for evidence other than the words of the poem, here are my thoughts:
- The whole theme of the poem is that even the mighties people and empires fade away. This theme is portrayed much more effectively if Ozymandias is a despot.
- In addition, he must surely have been a despot because who else, in that time and place, would have had a huge statue made? Making a huge statue takes resources (like building the pyramids did) and imply that the person who had it made in his honor was willing to take resources from his people to glorify himself.
I want to point to a third element in Percy Bysshe Shelley's poem "Ozymandias" that reflects the idea of the despot, the lines toward the end of the pome that has stuck with me for well over twenty years:
"My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!"
Nothing beside remains.
He saw himself not just as a ruler but as a ruler of rulers, as one who could make the mighty despair. Yet all of this man's achievements -- through which he had hoped to secure his immortality -- have been reduced to dust by time, and his posturing now looks like sheer and foiled narcissism.
That doesn't really answer your question, though. Are you looking for a biographical reason? A reason tied to the Romantic movement? Or what? You may find some answers in the links given below.
One of these links writes that the poem is connected to at least two ideas that may match your interest in tracking down the reasons for portraying Ozymandias as a despot:
1. 19th-century discoveries of past civilizations and the "great man" theory of history
2. Shelley's awareness of the ephemeral nature of his own achievements (in art and philosophy) or perhaps even his own flawed aspirations to be the "great man" and "arbiter of morality, genius, and political order"