There were mixed opinions today on what impact the marriage would have on Miss Bhutto's campaign to unseat Mr. Zia, who has promised elections in 1990. Her press secretary, Baseer Riaz, said the traditional marriage would shield her from the criticism of Islamic fundamentalists, who have attempted to portray her as an advocate of modern, Westernized ideas about the role of women.

But some experts in Pakistani politics said the alliance with a ''feudal family'' like the Zardaris could alienate young left-leaning members of the Pakistani People's Party.

In her message to party supporters, Miss Bhutto stressed what Mr. Riaz called the ''sisterly image'' that has been one of the big drawing cards since her triumphant emergence as a political leader in April 1986 after years of house arrest and exile. ''I am your sister and will always be your sister,'' she said in a prepared statement sent to Pakistan on Wednesday. Zardaris Initiated Talks

The Bhutto and Zardari families were assembling in London today and were awaiting the arrival of the prospective groom's father, Hakim Ali Zardari. He was a supporter of Miss Bhutto's father, the late President Zulfikar Ali Bhutto, who was overthrown by President Zia in a military coup in 1977 and hanged two years later.

The elder Zardari is now province leader of the Awami National Party, an anti-Government alliance seeking to link Pathan and Baluch tribesmen. The Zardaris were originally from Baluchistan, a neighboring province of Sind.

By all accounts, the Zardari family initiated the negotiations about a year ago by contacting Miss Bhutto's aunt. The discussions became serious in February in talks between Miss Bhutto's mother, Begum Nusrat Bhutto, and Mr. Zardari's stepmother.

The two older women were present when the couple met here for the first time five days ago.

Miss Bhutto noted that in a strictly arranged marriage of the traditional sort, she would not have been offered the chance to meet and approve the prospective groom. ''This was not the case,'' she said. ''I did meet him, and because I felt he's nice and had a sense of humor and he seemed to be a tolerant person in that he could handle having a wife who had an independent career of her own, I thought it was wise to accept the proposal.''