Put any subject in front of is/are and "a few sandwiches short of a picnic." He is a few sandwiches short of a picnic. She is a few sandwiches short of a picnic. That guy is a few sandwiches short of a picnic. They are a few sandwiches short of a picnic.

It's implying that someone is not too intelligent, or perhaps even a little bit crazy.

To expand a bit . . . This construction has become widely used to mean not terribly intelligent. One of the first examples was: A few bricks shy (short) of a full load. OR A few cards short of a full deck. Now people can make up their own versions using the same construction.