Ozymandias is definitely not Shelley's idea of a good ruler. Have a close look at the poem and you'll see that the persona is describing the ruins of Ozymandias's statue in the desert. Shelley shows how powerless and unimportant King Ozymandias seems to an observer now by the way his "wrinkled lip" and "sneer of cold command" are "Half sunk" in the sand, and the way the "Two vast and trunkless legs of stone / Stand" without a body on top of them any more.
Most striking is the way Shelley quotes Ozymandias's engraving on the stone: "My name is Ozymandias, king of kings: / Look on my works, ye Mighty, and despair!" The arrogant language contrasts hugely with the emptiness which Shelley goes on to describe in the final lines:
Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away.
Notice the way that Shelley uses alliteration in the last couple of lines to create a dreamy, thoughtful impression. He is making the point that Ozymandias was deluded about the extent of his power. No one is around any more, including Ozymandias himself, to be impressed by his works.
However, we are looking on his works now, but with a different set of emotions, perhaps sadness at the way human lives are quite short or reflectiveness about the ways humans try to leave a legacy after their deaths.