If Rosalynn Carter and Margaret Trudeau took time out to talk about fashion - and it is very unlikely that they did - they would find they had a lot in common.

Both like clothes but don't like to harp on the subject, particularly in front of the press. Both have an independent style and opt for the comfortable and casual, including jeans for the most informal occasions. And just as Mrs. Carter brought her sewing machine to the White House, Mrs. Trudeau has one in her residence at 24 Sussex Drive in Ottawa.

Mrs. Trudeau, who occasionally hops on her bike, wearing jeans, and goes off to shop with a similarly dressed Royal Canadian Mounted Policeman trailing her on a bike, likes to browse unnoticed through the racks. She makes her selections at boutiques such as Josee Les Jardin, John Warden or Rive Gauche in Montreal or the lien Holt Renfrew specialty shop.

"In the beginning she bought a lot of clothes from Canadian designers," observes Warden, a designer of ready-to-wear and lingerie who has his own boutique. "But now she seems to prefer others. She may be shopping on Seventh Avenue." Later he added sadly. "It's really too bad. She could do for Canadian fasion what Mrs. Onassis did for the Americans."

There are French designer labels in her wardrobe, including Yves Saint Laurence and Chloe, and outfits from Italian designer Andre Laug, but for very special occasions she makes her own dresses. She designed and made her own wedding dress (and baked the cake too) and created the wedding dress as well for the nanny to her children, Mary Alice Mullally.

Designer Warden, who has not been Mrs. Trudeau in the dress worn to Monday's state dinner at the White House, terms the choice "a little on the classic side" but believes it was "just as fashionable as a long dress."

Wini Rider, Fashion editor of the Montreal Gazette, says Mrs. Trudeau's choice of a short dress for a state dinner probably grew out of the much touted informality at the White House. "We all have been reading how low-key the Carters have been so perhaps she was responding to that."

There is little written about Mrs. Trudeau's clothes in the Canadian papers, not only because she doesn't want it discussed but also because people are not that concerned, generally, about what she wears, according to Carolyn Weiner of Holt Renfrew. "She's a very private person and she does her own thing. I admire her for that," Weiner adds.

One Montreal boutique invited press coverage of Mrs. Trudeau's shopping excursion to the store and later found all the clothes she had purchased returned to the shop. And she has not returned there since.

Several American designers have criticized Mrs. Trudeau's choice for Monday's dinner and Laetitia Baldridge, former social secretary to Jacqueline Kennedy Onassis when she was the First Lady, agrees. "Unequivocably it should have been a long dress," says Baldridge. "It's a most formal and dignified occasion."

It's hardly the first time Mrs. Trudeau's independent style has been criticized. Mrs. Trudeau told one caller to a radio station, where she was answering questions about having worn blue jeans and a Liberal Party T-shirt on a visit to Cuba, "I dress and act like a lady most of the time. I have some beautiful lady-like clothes."

To another caller she announced that she no longer wanted to be simply "a rose in my husband's lapel." Then she added, "If you rely completely on protocol, you can become a robot."