The Soviet news agency Tass delivered a stinging attack today on President Reagan's nomination of Kenneth L. Adelman to head the U.S. Arms Control and Disarmament Agency.

Tass called the president's action a "farce" and described Adelman as "absolutely unprepared" for the job.

The attack on Adelman, who was selected to replace Eugene V. Rostow, was highly unusual. The Russians normally do not criticize presidential nominees before they are confirmed.

There was speculation among informed observers here that the extraordinarily vitriolic comments could have been designed to strengthen Adelman's chances to win Senate approval. According to this theory, it is Moscow's view that his confirmation would help Soviet efforts to raise doubts in Western Europe about Reagan's intentions on arms control.

Tass likened Adelman's nomination to that of William Clark, Reagan's national security adviser, recalling that Clark initially had been confirmed as deputy secretary of state after a hearing at which he "openly admitted that he did not know the field where he had been appointed to work."

"History repeats itself," Tass said, "the first time in the form of a tragedy and the second in the form of a farce."

If Adelman is confirmed, Tass continued, "the appointment to such responsible posts of people who are absolutely unprepared for handling such jobs threatens millions of people living in the world with tragedy.

"Undisguised ignorance, which distinguishes some highly placed officials of the Washington administration, combined with the military power in their control, is a terrible threat to peace and the world."

In comments apparently directed at Western Europe, Tass asserted that the "dilettantes" in key positions in the Reagan administration are causing concern "first of all" among "the nations of the European continent, which the Pentagon strategists openly view as the most probable theater of waging their limited nuclear war."