You think? Levels are harder to draw with non-casters, since 'best swordsman in the world' means a very different thing in a world where everyone else is 2ne level than one with more of a range, for instance. They did tangle with some serious stuff, though, up to and including nominal deities.
In any case, he's the archetypal rogue (you can point to a lot of the early Thief oddities, like the affinity for languages, minor use of magic (reading scrolls) and preference for the sling, as Mouser-inspired), whatever level you might think is appropriate, while Batman isn't even in the same genre.
Having read the first two books of the series, Fafhrd and the Grey Mouser have a low-level feeling to me. They're competent swordsmen and thieves, but not larger than life like, say, Conan. Of course it's subjective, and I imagine I might think differently if I'd read the later books. Mind you, the Mouser wouldn't be a single-classed rogue or thief in any existing edition of D&D, anyway. He needs some personal combat expertise, which the class has never really had. Well, I suppose 4th edition could pull it off, since there are feats that can reduce the reliance of a rogue on flanking for gaining combat advantage.
As far as Batman goes, I don't think it's fair to dismiss an example just because of its genre. D&D has always drawn inspiration from many different sources.