Eh, I don't have a problem with Clark being unworthy, "worthy" isn't a remotely objective term and there are traits the obvious wielders (Thor, Cap, Bill, Diana if you're hopping the fence) have in common that they don't share with Clark - a willingness to kill when necessary, for one (a truly massive one at that), and a sworn dedication to a social hierarchy and the corresponding obedience to one's superiors, for another. If Mjolnir considers the virtues of a "worthy" possessor to include one who obeys his master and has the strength of heart to slay great foes, you get soldier Cap, warrior heir to Asgard's throne Thor, warrior guardian of his people Bill, warrior champion of Athena Diana, but not freelance technical pacifist Clark.
Or maybe Mjolnir wants avatars who stand for something bigger - two gods who incarnate storms and truth, a guy who's the living embodiment of an entire culture, and the transfigured repository of an entire species' hopes would do it, but a guy whose greatest strength is that beneath his powers he's a perfectly normal average human wouldn't cut it.
There are lots of totally logical ways to draw the line that exclude Superman but not other people who've held the thing under less exceptional circumstances; the problem is when you try to say that only Thor is naturally worthy, which is demonstrably not true and never has been, and makes the whole concept of "worthy" meaningless in the first place (since it now doesn't mean anything about Thor except that... he's Thor, while before it said that he was noble and righteous and surpassingly awesome by some external measure).