When the New York Daily News ran a headline referring to the leader of Nicaragua as "Fatso Somoza," Ian MacKenzie wrote an angry letter protesting the newspaper's "personally abusive" tactic. When Time published what MacKenzie considered a false story of a government massacre in a Nicaraguan village, he complained to the National News Council in New York. And lately the Carter administration's withholding of economic assistance to Nicaragua - and postponement of military aid - has drawn MacKenzie's ire.

Ian MacKenzie is paid to be angry when Nicaragua or its president are maligned in a public forum. From a small office in the National Press Building, he runs the Nicaragua Government Information Service.His client is not exactly a public relations man's dream: lately criticism of the regime of Anastasio Somoza has reached new heights, with Carter's human rights policy focusing attention on what some maintain is a corrupt, brutal government whose well-being depends largely on trade with and support from the U.S.

MacKenzie complains that Americans are never told about the stability Somoza brought to his country, about the rural electrification, the upgrading of education.

"The president is totally different from what people think," says McKenzie, who blames vocal rebels for the distorted image of Somoza. "He is loyal and strong with his friends, compassionate with his enemies.

MacKenzie, suave, white-haired, coy about his age (he's in his fifties), was born in Argentina of Scottish parents and traveled the world as a consultant for a London electronics firm. Two years ago he began working as public relations counsel to Somoza.

"The president is totally different from what people think," says MacKenzie. "He is intelligent, most capable, warm-hearted. He is loyal and strong with his friends, compassionate with his enemies."

MacKenzie reaches for a copy of Nicaragua's opposition newspaper, La Prensa. Its publisher would like to be president, and he is pictured on the front page speaking to a political rally, shaking his fist. If Somoza were as ruthless as some say, would he permit such opposition, MacKenzie asks?

"By the sheer law of averages, Somoza has had to have done some good," MacKenzie says. "Even Mussolini did some good for Italy."