Not sure what the OP means, but perhaps it has something to do with a higher incidence of war and such during the reigns of queens than kings.
But I would be careful calling monarchy despotic as if it were synonymous. Democracies can be far worse. The mob is dangerous and easily shaped through propaganda (as we are witnessing). You can hide from Nero, but the mob is everywhere. Besides, feudal dynastic monarchies aren't Enlightenment despots. Feudal lords, the nobility, the Church, and having to consider the consequences of your actions for yourself during a long reign (vs. 4 year term) as well as the reign of your heir all put limits on the actual power a king has.
I’m all for dispelling Whig propaganda about the European Middle Ages, but I’m not sure medieval monarchies and contemporary democracies can compared in this way.
As you mentioned, medieval societies were heavily variegated, religious, and did not have the means to manipulate nature in the way we do today, all of which were practical constraints on a wicked or despotic monarch. These constraints are now gone. I don’t think you could hide from Nero (or King John, to stay more overtly medieval) today.
Also, mobs were not absent from medieval society (think of the antinomian, flagellant movements in Germany, or various anti-Jewish outbursts). They’re likely to be with us to greater or lesser degree until the end.
My view is that for those of us concerned by the trajectory of modernity (Christian or not), the task is to better understand the phenomenon of totalitarianism, which we will have to contrast against a recovered, “re-realized” understanding of authority (as freedom from arbitrary power). But I don’t think medievalism, or pitting democracy vs monarchy will get us there.
There is a traditional Chinese view that women in power promote instability. (And that instability is bad.)
It does appear to be true that female rulers reign during times of increased instability, but it's hard to draw the distinction between female rulers reigning during instability because those are the only circumstances in which they can take power vs female rulers intentionally (or accidentally-but-predictably) creating instability.
And then you can draw another distinction between female rulers like Wu Zetian who have themselves crowned, explicitly making a change to the system, or female rulers like Cixi who exercise power they don't formally hold, which is working within the system.
