HIS DIPLOMATIC appointments, Jimmy Carter promised in his campaign, would be made "exclusively on a merit basis, in contrast to the political patronage that has characterized appointments under this administration." Then why is he naming to the Moscow embassy an amiable and successful retired business executive, Thomas J. Watson Jr. of IBM, who seems peculiarly unqualified for that post?

Mr. Watson wants the job and he enjoys the confidence of the secretary of state, but otherwise his credentials for this demanding ambassadorship are not readily evident. He is a distinguished and public-spirited citizen, but his proven talents appear to lie in fields remote from Moscow. He is likened by his friends to W. Averell Harriman, a political appointee who was wartime ambassador to Moscow. But Mr.Watson lacks Gov. Harriman's close personal relationship with the president, his deep political experience, and his long previous familiarity with the Soviet Union. There is a place in foreign embassies for the talented amateur: The Foreign Service cannot claim a lock on the post. Mr. Watson does not fill the bill.

The most puzzling aspect of the appointment, however, is what it reveals of the Carter administration's sense of Soviet-American relations. The administration has taken pains to suggest that the Soviet Union is run by old and ailing men and that it is essential for the United States to understand and cultivate the new leaders sure to arrive on the scene soon. This is so. But that argument for posting a savvy hand, not a new boy, in Moscow, which is, after all, not the kind of post where any kind of political appointee could be parked out of harm's way.

Alternatively, Mr. Carter, realizing that home support for his diplomacy is thin, might want to send an ambassador who would be useful to him in the political arena. But sending someone with a background as a multinational corporation executive gives the president no visible help here. Nor does trade loom so large as a factor in the immediate Soviet-American future as to justify the choice of an envoy on that ground alone. It is even conceivable that a president might want a skilled negotiator in Moscow. But that would require, presumably, someone who had demonstrated an interest in substance in the past. The appointment simply does not meet the standard Jimmy Carter set in his campaign or the standard for what is needed now.