The poem's central line that establishes the theme of power:
"My name is Ozymandias, king of kings:
Does the "king of kings" part happen to be a subtle reference to the English papacy by any chance? I don't know much about the European church system (not that I'm supposed to know anyway, given that I'm from South Asia), but it did come up in a few chapters of our History textbook. There were only 2/3 paragraphs at best on that topic and from what I've gathered, the Church used to wield more power than any king in Europe (I've no idea how much geographic area it spans). So basically the pope (the head of the Church) ruled over all the kings and could arguably be called the "king of kings". Sorry if I get anything horrendously wrong.
So, is Percy Shelley condemning the extent of the Church's power here? I'm aware this theory/interpretation is far too far-fetched to be what Percy really intended to convey, but it does explain the line somewhat.
It seems that there has been an misunderstanding. All the answers below address whether the pope, at any point in history, used to be referred to as "king of kings" either informally or as an official title. But I never did suggest such a thing in the first place. I meant that because the pope had once been powerful I suppose calling him "king of kings" would not be too unreasonable. I was merely inviting the idea of the pope being a possible candidate for the "king of kings" Percy talks of here. I certainly didn't mean that the pope really could have ever been called so.
Again, don't get me wrong: I never claim the Pope is necessarily a good candidate for Ozymandias. I was simply suggesting the notion of that being possible to some extent. The very purpose of this question was opening this up to more interpretation. Note that I also say: "I'm aware this theory/interpretation is far too far-fetched to be what Percy really intended to convey [...]". I've admitted myself that I don't find the whole thing being very likely either.