It's not like Japanese doesn't have the concept of "ou/joou" for sovereign monarchs. Tennou is just specifically the Japanese one, and yes, it's translated as Emperor, but this is more of a prestige thing than any meaningful difference.
Traditionally, an Emperor is either someone who is ruling over other sovereign monarchs, conceptually a "King of Kings" or at least of multiple royal crowns. Or someone who rules by some sort divine authority beyond your average king, like the "Holy Roman Emperor" being crowned by the Pope.
But in practice it's basically a matter of prestige and simply of how you translate titles (how do you translate sultan, how do you translate czar, etc.).
Queen in this context makes the most sense to me since I doubt there's a variety of Asian sub-vampire-kingdoms, nor do I see how there could be some divine aspect to her "rule".
