I strongly disagree. I like Civ6 leaders a lot! They are much better than Civ5 ones.
When you try making "realistic" people and their animations, any unnatural things in them look much more unnatural. That's called "uncanny valley" and Civ5 is much closer to it than Civ6.
Some leaders look ok. But Khan should not look like a funny guy because he wasn't like that. So his looks is not historically accurate. Even carticatures can look dangerous or evil etc. So it's not even about the artstyle, which is a matter of taste.
As for realism in games, the current technology handles it pretty well. Though, I don't care about realistic graphics in games so much.