life-clown

When one has previously been the "class-clown" who, in later life, still maintains the same essential attitude. That being to laugh and joke about anything and everything.

That guy keeps joking whenever his boss is trying to make serious decisions and policy. He's such a freakin' life-clown!

That life-clown won't take anything serious!

That guy makes a joke about everything, he's such a life-clown.