I’ve always attributed the clumsy, bumbling, cowardly, to him trying to hide any ideas that he’s Superman. He wants to keep his identities unknown, and his decision to be without a mask as Superman means he needs to keep people he knows from seeing through it.
So going out of his way to draw attention to himself with a Steve Urkel act is supposed to draw attention away from himself?
Sorry, but I’ve got a better idea; no one would suspect Clark is Superman because Clark is just so damn average in appearance and mannerism that no one would even begin to suspect that he’s the larger-than-life red-blue-and-yellow clad Kryptonian demigod.
In other words, he’d have no reason to “play up” Clark since Superman himself is already innately played up.