What KIND of Dictator?
The problem is that ANYBODY can be a "dictator" over, well, something. Your boss could be a dictator, yes? The coach of a kid's soccer team could be a dictator. The lady who runs your local charity could be a dictator.
EXACTLY what do you mean by "dictator"? If you mean someone like Hitler or Stalin, then you're talking about the Emperor, because their aim is conquest and control over an "Empire."
But if you're talking about dictating what people believe, spiritual control, THEN you're talking the Hierophant. In this respect, he is like any church leader or elder who has absolute control over the church's teachings, beliefs, etc.
Depending on how much power the church has, then, yes, he could very well be a Dictator. In Utah, the Momon Church leaders have a great deal of power. But in California, they don't have much power, do they? In a Catholic country like Mexico, the Catholic church has great sway over political leaders and the people--it dictates. But in England it has no sway.
So whether the Hierophant represets a spiritual dictator, a church leader who has as much if not more power than the political leader of that country, all depends on circumstance. He can certainly be a dictator in the sense of "cult leader"--but he's unlikely to be a dictator in the sense of leading armies and conquering lands, not unless he stands for spiritual leaders in a holy war.
So keep that in mind. It's not that he stands for "dictator" but rather that he stands for a certain TYPE of dictator. There are plenty of other cards that could indicate other types of dicators--from the Emperor standing for a more Roman type dictator to the Empress--that obnoxious woman who runs the local charity like a female Hitler