Designer Dominic Rompollo was just a design student at the time of John Kennedy's Inauguration in 1961 and couldn't recall the clothes the First Lady wore then. But others were remarking yesterday on the similarity of Mrs. Carter's (and Mrs. Mondale's) outfit at the swearing in - stand-up neckline, fitted through the body, and an easy skirt. That was a Jacqueline Kennedy Onassis signature that was watered down for Pat Nixon, and reiterated this year. It is a silhouette that works well for women with long wastlines and shorter legs - thus the echo of a swearing-in 16 years ago.

It didn't occur to Rompollo to put fur on Mrs. Carter's Inaugural Day outfit - or on any of ther five outfits he has done for her. "I'm sure it would have offended her ecologically," said the designer. She had a fringed shawl made to match her coat, but didn't use it.

The Republican cloth coat has apparently given way to the Democratic mink.

At Wednesday night's Kennedy Center gale there were vintage mink stoles, some eqully nontrendy light colored, even white, mink jackets, and deluxe natural dark, dark minks. The mink business is flourishning because with the high price of quality cloth coats, a mink coat is not a bad investment (and mink is in no way endangered; it is bred for such purposes).

Fashion gave away to function for Inauguration-watchers yesterday. The rule was, whatever you own that's warm, wear it. Ski masks, hooded parkas, mufflers, hiking boots and snow-mobile mittens were apparent favorites on the surface; long johns and turtlenecks played their warm-up role underneath.

Among the spiffier observers were the ambassadors' wives, who were hustled into the press section when there was a snafu over their reserved seating. Many were in furs, with mink the most popular. Mrs. Azraai Zain, wife of the ambassador from Malaysia, wore a white knitted cap and her fur coat over long native dress. "It's the warmest think I own," she said.

Only lady with fashion smarts in the new administration is Margaret (Midge) Costanza, Carter's assistant for public liaison. Wednesday night she was in a black dress with decolletage keyhole cutouts framed in rhinestones. "Not the sort of thing I'd wear at home," she admitted, and for last night's parties she expected to wear a snappy chiffon top over crepe by Julio.Before leaving homd, she spent six hours choosing 14-plus outfits to carry her through the Inaugural celebration and beyond.

"I just felt it was a good occasion for getting out of my jeans," said Elizabeth Ray, last years's political bomb-shell, of the Carter swearing-in. She was wearing a Cardin white felt hat, mink coat and sunglasses and had sweet-talked her way past Capitol police into the press area, where she gave away autographs. She's now a drama student at American University. She giggled as police pointed out to her another, similarly dressed woman whom many were photographing, thinking it was Ray.

At the Mondale reception on Wednesday, there was a lesson to be learned from the large Minnesota contingent on how to cope with the cold. Fur-collared coats, that's how, clearly a favorite in Minnesota. The collars are mostly real fur but occasionally fake and cover the ears when the wind starts to blow.

Alisa Moses has obviously picked up some merchandising tricks from her grandpa. (He's Jack Moses, owner of Jason's in Americus. G2., a favorite shopping spot for the the First Lady. Alisa made a deal with a vendor selling Carter knitted caps at the Georgia party Wednesdy night at the Sheraton Park - $1 cut on each hat sale she made among her grandfather's friends. Total take for the evening was about $10, including three hat sales to designer Dominic Rompollo.