California Gov. Edmund G. (Jerry) Brown Jr. today praised SALT II as emboyding the spirit of "the era of limits," but criticized the MX mobile missile program as a " $30, $50 or $70 million boondoggle."

Brown, who belatedly entered the 1976 presidential primaries with some success, has not delcared as a 1980 Democratic presidential candidate. But his maiden speech on the strategic arms limitation treaty (SALT II) and his first foray since 1976 into national policy did nothing to change the view that he will announce his candidacy later this summer.

Brown spoke at the University of California campus here to about 75 students at the U.S. Student Pugwash Conference, the theme of which was borrowed from the international Pugwash Conference organized in 1957 in Pugwash, Nova Scotia.

Later in the day, in comments to a Washington Post reporter, Brown said he hoped his support of the SALT II treaty "would enhance its chances for Senate retification."

The MX program proposed by the administration would provide for deployment of 200 additional land-based nuclear missiles. The basing system hasn't been decided on, but the most publicized idea is to deploy each nissile along a 20-mile track, with 40 stations per track, thus keeping the missiles' locations secret. The theory of the mobile warheads is to frustrate the theoretical Soviet capacity for a first strike that could destroy 90 percent of the American ICBMs in fixed silos.

Brown criticized the MX because it would not be completed until 1986 at the earliest, and he claimed that the United States will be most vulnerable in the early 1980s.He also made it clear that he would rather have the money spent in other areas, notably energy research.

Brown's speech signaled an attempt to outflank Carter from the left on the arms race issue. Tom Hayden, a former campus radical now active in California Democratic politics, accompanied Brown to the university.

Brown derisively referred to the MX as "a mass transit for missiles," and lambasted "spending billions for weapons we will never use."

He said future threats to the United States and the West will not come from Soviet weapons, but from Third World peoples imbued with nationalism, beset by unmanageable birth rates, plagued by a lack of resources and possibly possessing nuclear weapons.

"Pride comes before a fall," Brown said. "We are very puffed up with our power; the Soviet Union is very puffed up with its power. But when I look at a world with 4 billion people . . . with plutonium in the hands of people less responsible than those who have it now, we won't be secured by increasing overkill. We will be secured by a more equal distribution of world opportunity."

Brown said deployment of a mobile missile system would "force the U.S.S.R. to respond in kind." The Soviets have produced and tested a medium-range mobile missile, the SS20, but not a mobile ICBM.

Brown also said MX would "use up very scarce capital which could be used for building up energy bases, solar, geothermal, liquid fuels, electric cars, rapid transit and insulation for homes."