Vice President Mondale's relatives have descended on his Cleveland Park residence for the inaugural restivities. How and what to feed them looked like it might mushroom into a major problem until somebody noticed that a lot of Mondale neighbors were descending on the house begging for "something to do."

"Let them cook," said somebody, so Gretchen Poston, one of Joan Mondale's closest friends, started calling neighbors and set up a "carry-in" dinner operation. Each night a different friend or neighbor provides the home-catered meal.

A list of cooks and their menus is pasted up on the cupboard over the sink in the Mondales' kitchen. Any of the visiting relatives - who have come from as far away as Norway - can find out what's for dinner just by looking.

The "carry-in" service began on Sunday with a dinner of brisket of beef, potato broccoli casserole, cranberry mold salad and nesselrode pie provided by friend and neighbor Margery Elfin.

By this morning the Mondale and Adams (Joan's family) clans will have eaten their way through ham, fried chicken, chicken and sausage casserole and veal stew. Leftovers make a marvelous lunch, one of the relatives said.

The climax to the home-catered meals comes today when 50 Mondale relatives will tour the new Vice-presidential mansion, Admiral's House, sip coffee and nibble on pastries made by Barbara Rothkopff.

The system works, according to Joan Mondale aide Bess Abell, because it isn't at all democratic. Which is the same season Joan Mondale once gave for why the food co-op she organized has worked so well: no orders, no returns and no complaints.