Around the U.N. Plaza, some New Yorkers are acquiring a reputation for being Ugly Americans.

Ambassadors to the United Nations, counsuls general and other foreign mission officials are being snubbed regularly by owners of fashionable cooperative apartments on Manhattan's East Side, ostensibly because of the potential for political demonstrations and peculiar legal questions stemming from their diplomatic immunity.

But the real reason, some U.N. officials believe is snobbery and fears of imagined lifestyles centering on interminable parties, elevators and lobbies crowded with security men, flagrant violations of parking regulations and other irritrations to the genteel milie of luxury apartment living.

On Park Avenue, Beekman Place, Sutton Place and other fashionable neighborhoods of New York City - particularly in the older apartment buildings - just the notoriety of being a diplomat may be enough to cause a simple majority of the cooperative owners to turn thumbs down on the sale of a luxury unit costing $500,000 or more. Co-op apartments - apartments owned by their residents - in New York commonly have bylaws which require all sales to be approved by the majority of the occupants.

"It's a disease. Every government, every country, every ambassador - whether it be Third World or Western European - has great difficulty in buying apartments in New York," com-New York City Commission for the plained Frances Loeb, head of the U.N. and Consular Corps, a government agency that acts as liaison to the world organization.

Obviously angered, Loeb, whose duties includes assisting diplomats in relocating here, said, "New Yorkers have no shame. It's all well and good to say they like foreigners because it adds international flavor to the city, but when it comes right down to their own home, they're very parochial about it."

The problem of cooperative apartment owners blackballing foreign nationals surfaced recently when Prince Saud al-Faisal, Saudi Arabia's foreign minister and heir apparent to the Saudi throne, encountered opposition when he tried to buy a luxury 13-room apartment on Park Avenue at 66th Street.

The dozen owners in the building reportedly quarreled among themselves over whether to approve the sale of the fifth-floor apartment to Saudi by the present owner, Bruce A. Norris, president of the Detroit Red Wings hockey team and heir to a grain fortune.

While no opponents to Saudi have reportedly quarreled among them publicly identified themselves or explained their opposition, some owners said privately that fears have been expressed at meetings about disruption of the building's tranquility.

A vote on whether to approve the prince, a Princeton graduate who reportedly wants an apartment to stay in when he heads his country's delegation to the United Nations, was put off by the owners indefinitely.

U.N. officials, real estate brokers and officials of the City Commission say, however, that the controversy over Prince Saud is merely a symptom of a larger problem.

For years, they said, foreign diplomats have been rejected by some apartments because of their diplomatic immunity which could allow them to evade sutis for breaking contractual agreements, such as monthly building maintenance charges. Cooperative apartments charge such fees, often $25,000 or more a year, for building-wide services and upkeep.

Also, they said, co-op owners have told of fears about abuses of the use of diplomatic license plates on surrounding streets, large social functions and constantly milling enourages of personal aides.

Loeb, however, termed the fears as unjustified, noting that diplomats and their families generally lead unusually quiet lives, and that the city does not permit DPL plate parking on those residential streets.

"They [the owners] are trying to ationalize. I feel it's part of a coverup of their real objections," she said, implying social prejuice." It's not Saudis, Syrians or whatever. It's all diplomats. It's being anti somebody who has notoriety," Loeb added.

Vetoes of diplomats in recent years, she said, include top U.N. representatives from countries as diverse as Turkey, the Netherlands, Israel, Belgium, Spain, Argentina, Venezuela, and Italy.

Five years ago, after a wealthy Canadian woman died and her East End Avenue apartment went up for sale, the Canadian ambassador was unable to buy the 22-room apartment, even with the personal backing of then mayor John V. Lindsay.

When Robert Fack, chief Dutch representative to the United Nations, tried to buy an apartment at 1 Sutton Place in 1972, the board of cooperative owners vetoed it 6 to 0, despite a fervent plea by Loeb, a commission official recalled.

Loeb said even Laurence Rockefeller could not persuade the board of his Fifth Avenue cooperative to reverse itself once it rejected a Venezuelan ambassador several years ago, and that the owners of an apartment at 760 Park Avenue told the commission flatly once they did not want foreign diplomats.

Real estate sources said that, generally speaking, there has been a steady increase lately of foreign nationals buying real estate here, both for speculative and residential reasons, but that they have tended to go to newer buildings where they are less likely to encounter opposition.

Many of them have moved to new luxury buildings like Galleria on East 57th Street or the late Aristotle Onassis' 52-story Olympic Tower next to St. Patrick's Cathedral, where approximately $700,000 buys a four-bedroom penthouse with a private sauna and internal elevator. About half the purchasers at the Olympic Tower are foreign, and sales brochures are printed in French, German, Spanish and Japanese, as well as English.

"These buildings have limited hotel-type service, the sort of thing that appeals to foreign buyers," noted Brewster Ives, partner in one of New York's largest real estate firms. Also, he said, the kind of opposition Prince Saud faced is more rare in the newer buildings than in the "older, more stuffy cooperatives, where they are problems with older board members."

Officials of foreign missions generally are reluctant to talk about the difficulties of buying apartments, for fear of embarrassing their governments and creating a protocol flap, commission officials noted, and most of the cases receive no public attention.

James O'Rouke, an attorney for Norris, said Prince Saud declined to comment on the dispute at 640 Park Avenue, but added that the mouth-long delay by the cooperative board "seems like an awful lot of wheel-spining, even for a Saudi Arabian prince."