Here he is in "My X-Girlfriend’s Wedding Reception" — a 1999 romantic comedy:
He plays a rabbi named Manny Shevitz, and from that name I think you can infer the level of cleverness in the comedy.
Sanders is basically giving a speech, so you might think not much acting is involved, but in fact he doesn't milk the comedy material enough to fit the mugging from the actors playing the guests at the wedding.
But you don't need to be a great actor by movie standards to do the kind of acting needed in the presidency. Ronald Reagan was not that fine of an actor, but he played the role of President brilliantly.
Various politicians have done cameos playing themselves in a TV show or movie, and it took a little acting. For example, Condoleezza Rice had to say "'Mars Attacks' is awesome," in this montage. Watch for Michael Bloomberg — he's considering running for President — he's quite good, maybe at the Reagan level.