Of course mediocre and bad leaders make their mark on history. But Carlyle's Great Men theory is more about paradigm shifts that Great Men can will into existence, not just random noise they bring along. The problem with GM theory is that there is only a handful of examples to support it. Napoleon is one such example, and it was undoubtedly the inspiration why the theory was proposed in the first place. People were trying to come to terms with the fact that one leader can have such a dramatic impact on the entire world.
The historical setting at the moment of time matters a lot. Had the French Revolution not happened, he would've been yet another artillery lieutenant in the French army. In 1789 Jean Lannes was a dyer, Laurent de Gouvion Saint-Cyr was a paineter, Jean-Baptiste Jourdan worked in a clothing store, Jean-Baptiste Bessières was a barber, Guillaume Marie Anne Brune was a typesetter, Jean-Baptiste Kléber was an architect... et cetera, et cetera. But it happened, and lot of people got an impossible before chance to discover their dormant military talents.
Yes, but the point about Napoleon is that there was so much more about him than just military talents. The environment in which he flourished wasn't of his making, but he managed to grow in it and ended up impacting the whole world. Either directly by bringing the Napoleonic legal system to them with an army, or indirectly by inspiring or enabling nationalism, democratic ideas (power coming from the people, not deity), allowing the whole of Latin America to break free from the Spanish Empire by keeping the latter busy, etc etc.
Many people became successful militarily and even seized power afterwards during tumultuous times. Very few actually ended having such an impact worldwide.
And before any Brits come in with centuries old grudges, of course he did plenty of bad, most notably how he treated Haiti (which he at least acknowledged later in life).
Ah, yeah, he almost strangled Britain with his Continental Blockade. Would probably have been his greatest gift to the world, if he actually managed to pull this off. Oh well.
The same is happening in Ukraine right now.
Prior to the war, Robert Brovdi (Magyar) was a local businessman on a periphery of Ukraine. Now he is a commander of probably the strongest drone force in the world.
What is a historical theory anyway? These things aren’t for making predictions. So it seems hard to judge them the same way we’d judge a scientific theory, based on their predictive ability (too easy to cheat, since all the experiments you can make using history are concluded already, other than current events, which are only a tiny slice of history and we don’t really know which ones will be deemed memorable by the record).
I think the wrinkle of "men (or maybe women) at a particular hingepoint had their personal foibles that shaped history" is valid. There were structural forces that led to WW2 but Churchill, Roosevelt, Stalin, Hitler et al being their particular personalities absolutely shaped how it unfolded.
The same can also be said of e.g. the Cuban Missile Crisis.