Israel's smashing success against Soviet weaponry in Lebanon must be generating "some second thoughts in the Kremlin" about how its planes, tanks and antiaircraft missiles stack up against American ones, the nation's top-ranking military man said yesterday.

Gen. David C. Jones, chairman of the Joint Chiefs of Staff, who leaves office this week, also said before the National Press Club that the Israeli battlefield experience with U.S. weapons shows that "we don't have to be quite as pessimistc as we have been in the past about these systems."

Israeli pilots flying American-built F15 and F16 fighters claim to have downed 79 Syrian warplanes, most Soviet Mig21 and 23 fighters, but not the more-advanced Mig25 with improved radar. Israeli ground forces said they knocked out seven Syrian T72 tanks, the latest ones supplied by the Soviet Union.

While Jones did not go into it, much of the Israeli success in the air could be attributed to the U.S.-supplied AIM9L Sidewinder missile, which can be fired while opposing planes are head-to-head.

Soviet air-to-air missiles are not credited with this all-aspect ability.

Jones, in one of his last speeches in uniform, stressed the need to reorganize the Joint Chiefs of Staff before existing organizational snarls cause a disaster on some future battlefield. He favors a single powerful chairman rather than continuing the present committee system where the top officer of each service has a vote.

He cited the Vietnam war as fresh evidence that reforms to force coordination of military efforts are imperative.

"Vietnam is a real case of how not to do it," the usually diplomatic Jones said in a harsh critique of the military performance in that war.

"We lost sight of the objectives," Jones said. "Each service essentially fought its own air war. We lost the support of the American people."

Asked for his reaction to the recent Soviet pledge not to be the first to use nuclear weapons in a war, Jones said, "First, I don't believe the Soviet Union." He said he opposes a similar "no first use" declaration by the United States because it would undercut the deterrent to the Soviet Union starting a war in the first place.