Let's face it: most of his characters are rip-offs of (at least now) lesser known characters and just concepts
Captain America is an obious rip-off of Shield, so much so he literally carried his symbol as his original shield, couldn't be more obvious. Bucky is of course rip-off or Robin.

X-Men are rip offs of Doom Patrol, Brotherhood of Evil = Brotherhood of Evil Mutants, wow really imaginative. Niles Caulder = Professor X, etc

He was also not very imaginative in creating "new" characters like Spider-Man, who had web pistol and could climb walls just like Tarantula of DC Comics in 40's, and was dressed in similar outfit scheme at Cap and original the X-Men

And of course he wasn't first nor last, Batman is rip off from a bunch of things from movie The Bat, to Black Bat from which were also ripped off Daredevil and Two-Face

Or Superman which is clearly rip off of Gladiator, John Carter, admitted rip off of Doc Savage which in turn was rip off of Savage Gentleman.

Etc , etc etc/

So is Kirby a King because he done it most masterfully and was very successfully marketed by Stan Lee?