The name "Ozymandias" is the Greek transliteration of the "throne name" of an actual Egyptian pharaoh, Ramesses. Ancient historian Diodorus Siculus translated and recorded an extremely self-confident inscription at the base of a statue of Ramesses in Egypt; it challenged anyone who viewed the statue to try surpassing even one of Ramesses' works. By trying and failing, the viewer would come to realize how great Ramesses really was. Although Shelley's poem uses a version of the inscription recorded by Diodorus Siculus, the actual statue to which the inscription refers does not resemble the broken remnants described in Shelley's poem.
Basing his poem on the inscription alone, Shelley used the concept of a shattered, forgotten colossus to illustrate that Ozymandias’ conception of his glory had not one thing to do with being a good ruler. Instead, Shelley's scattered fragments of the supposed “King of kings” indicate that Ozymandias was the exact opposite: a good ruler would be remembered; a good ruler would not have to threaten those who might view his statue. The statue of a genuinely good ruler would show benevolence, wisdom, good will, and nobility, rather than sneering coldness and a love of instilling fear. And the statue of a good ruler, a ruler who earned his subjects’ love and respect by his careful and caring governance rather than by threats, would probably NOT lie shattered in the sands of a forgotten desert.
Shelley’s main observation here is the fragility of human accomplishment and fame (he is thinking of his own poetry, too.) But in the description of Ozymandias’ visage, as interpreted by the sculptor who “well those passions read,” we see a tyrant, a conceited, self-aggrandizing ruler who led by force, not wisdom. He apparently put a lot of his wealth and human labor into making a large statue of himself for posterity. So, no, I would not call him a “good” leader, and, speaking for the Romantic poet, Shelley, he didn’t think so either. This poem is about the irony of the inscription, seen from the time of the “traveler from an antique land.” Add to this the body of Shelley’s work, and there is no evidence that Shelley admired or promoted this kind of rulership whatsoever.