Civ3:
View attachment 700924
Civ4:
View attachment 700926
Civ6:
View attachment 700927
Bonus Civ3:
View attachment 700928
Of the four mainline Civ games with 3D leaders, three games had leaders that look anything but serious. You can call the style exaggerated, cartoonish, funny, kitschy or whatever, but it was definitely not serious. Civ5 was the exception with its leaders looking realistic and mostly having subdued animations. From what we've seen so far of Civ7, I definitely don't think it's less serious looking than previous games, it's probably a bit more realistic/serious than Civ6.
And Civ2, which didn't have 3D art yet, embraced the silliness so much it probably outsillies any later game.