Well, his clothing got fairly weird in the comics; I guess the idea was, hey, he’s fighting bad guys and has an alter ego; let’s have Captain Midnight dress more like a superhero.
So, are there more Captain Superheroes than Dr Supervillains? Or do they cancel each other out?
There was a Captain America villain named Captain Nazi, I believe.
DC’s lighthearted response to Captain America was patriotic do-gooder General Glory.
Kind of a different thing but in military installations, usually the Navy or Coast Guard, the Commanding Officer of the installation is often called “Captain”, despite his or her actual rank. For example, in “The Caine Mutiny” Humphrey Bogart’s character is sometimes referred to as Captain Queeg although his official Navy rank is Lieutenant Commander (two pay grades below what a real Captain is). Or an important business leader is sometimes called “A Captain of Industry”
Colonel Carol Danvers calls herself Captain Marvel. 
I thought she was Ms. Marvel.
Regards,
Shodan
She was. Carol Danvers switched her name to Captain Marvel in 2012.
Guardians of the Galaxy member Vance Astro took the name Major Victory after he acquired the shield formerly belonging to Captain America.
(Passed away in 2005. Here’s a picture of him with Stan Lee.)
I was in a superhero role-playing-game a few years back; my character was “Captain Doctor.”
A captain, Army type not Navy, is high enough to have some authority but not so high that he wouldn’t take place in front line combat.
For your enlisted types I give you that unsung vigilante: Corporal Punishment.
Ba-dum-CHEE. 
So you were playing both sides of the fence?
Grin! Yes, yes, and yes! The character was only an actor, pretending to be a superhero, with an inflatable “muscle” suit. Then he’d go and do a quick-change, and come back in persona as “Mentalissimo,” a mystic/mentalist. Then another change, into a tuxedo, and he was “Agent Spy.”
Gaw Dam I had fun with that character!
Don’t forget Professor Chaos (Butters) and his sidekick, General Disarray (Dougie) from South Park. 
But he really should have been Hauptmann Nazi. Or Hauptsturmführer Nazi, if he was in the SS.
That’s a Captain Marvel (DC) villain, actually.
Or from The Big Bang Theory, Captain Sweatpants!
I think the ambiguity of the terms has an effect:
Major Malfunction: A big screwup or a superhero?
General Malfunction: A widespread problem or a superhero?
Captain Malfunction: Oh, that must be a superhero.
(Lieutenant Malfunction: That’s redundant.)
AFAIK, the original Captain Marvel was the earliest use of the title in comics.
Here are all the superheroic Captains I can list from memory:
Captain Marvel (at least 7 of them)
Captain Marvel Jr.
Captain America
Captain Britain
Captain Canuck
Captain Atom
Captain Victory
Captain Glory
Captain Flash
Captain Comet
Captain 3-D
Captain Action
Captain Thunder
Captain Trips
Captain Planet
Captain Universe
Captain Metropolis
Captain Ultra
Captain Shazam
The parodies and humorous characters:
Captain Klutz
Captain Tootsie
Jughead as Captain Hero
Captain Nice
Captain Incredible
Captain Everything
Captain Carrot
Captain Avenger
Captain Justice
Captain Victorious
And the villains:
Captain Cold
Captain Boomerang
Captain Stingaree
Captain Nazi
Captain Rumble