There is also an obscure Quality Comics hero known as The Jester.
Also, the Joker’s Daughter. A horribly ill-conceived superhero concept, if you ask me.
Jimmy Stewart in “The Greatest Show on Earth” (1952) became a hero, (abeit, one with a sketchy past).
From short-lived BBC Three series “The Wrong Door”, episode five:
Sadly I seem to be the only person who found the show awesome and it was cancelled.
Did I mention Captain Goitre was played by Brian Blessed?
I give you Funnyman, atop his tiny unicycle while clad in oversized shoes and bowtie with baggy polka-dot pants, brightly-colored puffball-adorned vest, and, yes, “comic nose”. As the site explains, he’d dressed up as a clown for a super-hero-ey publicity stunt, but mistook a real crook for the hired actor and saved the day – without realizing it – by deploying the practical jokes as rehearsed.
When the facts were brought to his attention, he kept fighting crime and saving lives in costume – by which I mean trick suspenders, spring-loaded shoes, and an up-the-sleeve squirt gun – while beating James Bond by a decade or two in the car-with-oil-slick-and-ejector-seat department.
Depending on your definition of “clown”, there are quite a few of them still active today…in rodeos. You tell somebody thrown from a bull that these clowns aren’t heroes.
(For whatever reason, their title changed from “rodeo clowns” to “bullfighters” (although I would classify “bull poker” as the former rather than the latter) - in fact, for at least one year, the National Finals Rodeo had a “bullfighters” competition), which is strange as any actual fight between them and bulls is clearly one-sided in favor of the bulls.)
An interesting character in Saberhagen’s “Lost Swords” novels is “The Emperor”.
“The Emperor is often referred to as the Great Clown because of his penchant for wearing masks and allegedly playing pranks.”
Though whether he would count as a hero is up to the reader. Certainly he is very powerful and opposes the demons in that Universe.
One of Terry Pratchett’s books has a heroic clown, though it’d be a spoiler to say which one.
And in Order of the Stick, there’s Banjo the Clown, God of Hand-Puppets. And you could argue that Elan, his first disciple, is a clown, too.
Harley Quinn was a heroine for a while there. Also, the Golden Age Green Lantern’s wife, Harlequin.
There’s also the eccentric antihero Ragdoll, an acrobatic contortionist who’s been mistaken for a clown when in costume (since he wears a harlequin outfit and a ‘whiteface’ mask with a garish wig while moving like a circus performer). He’s a crook, but the comic-book supervillains he fights are worse.
What is this… some kind of joke?
I’m not sure if he counts, but how about Steve Ditko’s Odd Man, a superhero “garbed in a confused costume that would make a carnival clown blush with embarassment. His weapons were absurd – impossible! But somehow he became the terror of criminals, and everyone began to wonder … who is the Odd Man?”
As per the link, his rainbow-colored outfit (complete with weighted polka-dot tie and powder-and-smoke gloves) is arguably the most normal-looking thing in the guy’s headquarters; the Odd Man’s signature asset in the war on crime isn’t the gear that lets him spray the floor so crooks slip and fall, but the unnerving base of operations where he interrogates people by making them – well, not ‘frightened’ so much as ‘uneasy’. The guy’s strength is sheer damn weirdness.
I’ve long thought that there would be interesting possibilities for a story, or even a series, set in a drab dystopian world in which humor, color, laughter, etc. were prohibited by law; and a few brave individuals are clowns, who have to keep their clowhood secret and hide their wigs and makeup, but every once in a while they make raids of clowning as a sort of guerilla warfare against the enforced joylessness of the authorities.
I think you need to slap some copyright on this, before the people at Quirk Books see it. 
Yeah, something along those lines (and that may be where I got the idea in the first place); but I’d like to see it treated more “seriously,” with a fully fleshed-out plot (or series of plots), characters, and setting, rather than just an Orwellian (or Ellisonian) fable.
They did something along those lines with an alternate comic-book universe version of the Joker, who pulls off the secret-identity-alibi bit as three different women taking turns as the wig-and-clownface prankster zanily rebelling against conformity and authority figures.