Secretary of State George P. Shultz said yesterday that Walter F. Mondale's proposal to stop testing U.S. nuclear weapons temporarily would harm America's deterrence capability and thus would be as "reprehensible" as failure to seek arms reduction negotiations with the Soviet Union.

Mondale, the Democratic presidential nominee, said last Wednesday that if elected he would try to get nuclear arms talks back on track by seeking a summit meeting with Soviet President Konstantin U. Chernenko within six months and by declaring a unilateral U.S. moratorium on testing for that period.

But Shultz, interviewed yesterday on "Meet the Press" (NBC, WRC), reiterated the Reagan administration's frequently stated view that the best way to make the Soviets negotiate seriously is by demonstrating U.S. strength and resolve to deter aggression.

"I don't think it is a good idea to suddenly turn back from our capability to be prepared and to deter," Shultz said.

Later he added: "To stop your capacity to be prepared is just as reprehensible as to not seek negotiations for peace."

Referring to the desirability of summit meetings, Shultz said, "The point is not to have meetings, but to achieve something."

Under President Reagan, he added, the United States has achieved a great deal in restoring the strength and sense of purpose that he said were lost during the Carter administration, when Mondale served as vice president.

"We're trying to be prepared to deter aggression as well as prepared to negotiate for peace," Shultz said. "If the Soviets see they can't get their way by virtue of lack of willpower in the United States and the Western world, they may see that it's best to negotiate . . . .

"I can't make predictions about what might happen in 1985 or beyond. But the United States will be prepared."

Shultz said the United States will continue probing for ways to resume the arms talks and make other progress in East-West relations when he meets Soviet Foreign Minister Andrei A. Gromyko at the United Nations on Sept. 26.

U.S. officials who declined to be identified said last week that Shultz was approaching the Gromyko meeting with "a sense of realism and low-key expectations." But, the officials added, if the meeting goes well, it could lead to a second Shultz-Gromyko session or possibly even a Gromyko trip here to talk with Reagan.

Shultz, waving such speculation aside, said, "I just hope we'll have a useful meeting. I don't set any level of expectations . . . . If there's to be another meeting , that remains to be seen."

Asked about recent U.S. overtures to such Eastern European countries as East Germany, Hungary and Romania, Shultz replied, "We are glad to have better relations with the countries of Eastern Europe. We would like to see relations improve."

But he cautioned in respect to speculation that the administration hopes to drive a wedge between the Soviet Union and its Eastern European client states, "There isn't any big new campaign of any kind.