Definitely a case of sampling bias. We remember the exceptional leaders of the past for the simple reason that they were exceptional. But there have been many incompetent statesmen and monarchs who we tend to forget about or overlook.

Let's go no further back than WW1 and the last Kaiser of Imperial Germany, Wilhelm II. There's a silly saying that describes the three Hohenzollern Kaisers thusly: "Der greise Kaiser, der weise Kaiser und der Scheiße-Kaiser." Wilhelm II was the latter.

He frequently made public statements that contradicted his own government's policy and became notorious for inadvertently insulting foreign dignitaries. He dismissed Otto Von Bismarck (one of the greatest German political thinkers of the time), refused to renew the Reinsurance Treaty with Russia that Bismarck had designed (driving Russia into an alliance with France: one of the political alliances that contributed to WW1), and insisted that Germany pursue a policy of naval expansion (directly antagonizing Britain, who could have been a powerful ally).

I could go on, but the point is this: yes, there were many leaders from the past who were less-than-exceptional.