Hi.
It's NOT that I think Ben Grimm SHOULDN'T be able to lift it, it's that I think ONLY Thor should be able to lift it. It takes a special, noble, warrior spirit, someone with that special "Thorocity" if you will, to heft that mallet.
Personally? I think the list of people who CAN lift it is just too darn large as it is. To me, the only reason the Awesome Andy story works is because at the moment when Awesome Andy CAN lift it, he's a machine. He's no different from a winch, a crane, or a flatbed truck (and we have seen mechanical devices lift Mjolnir). It's at the moment when Andy is NOT a machine, when he's a sentient being with a soul-- that's when he can no longer keep it aloft.
I was at a Marvel meeting where a writer was unfolding the future events of one of their stories. They were doing a great job, everyone was enthralled, and then they reached a point in their story where they said, "And then (name of one of his secondary characters) picks up Thor's hammer!" The writer paused for dramatic effect.
And, after a beat, upcoming Thor-writer, JMS, said (and without any malice or meanspiritedness-- just a plain this-is-the-way-it-is kind of tone) "No, he doesn't."
And that was it. The other writer's story still happened. It was still unbelievably cool. But it just didn't have that story beat in it.
As a Thor fan, I appreciate that. You gotta keep that list down. Every time someone ELSE lifts up that hammer, Thor becomes a little less special. And you gotta watch out for that. (With the possible exception of Future Thors and Thor's descendants. To me, Future Thors work in that they have Thor's spirit-- a Once-And-Future-King-kind of scenario or, if you want to get all geeky about it, like Doctor Who regenerations. And with descendants, then it's a legacy thing like The Phantom-- and that works too.)
So, to answer your question, I don't think Ben could-or-should be able to lift it. Conversely, I don't think Thor could keep down ten pounds of Yancy Street knishes. So it all evens out. Each hero must be true to their own selves.