Mike Curb is a pleasant and uninformed millionaire with a keen interest in the presidential plans of Gov. Edmund G. (Jerry) Brown Jr.

Curb, who looks younger than his 34 years, is considered the nation's most successful record producer. He amassed a fortune estimated at $20 million while forming two record companies, salvaging another and lighting up the lives of millions of Americans with the voices of the Osmonds, Pat and Debby Boone, Shaun Cassidy and the Mike Curb Congregation.

But Curb's biggest hit has come in California politics. Because California voters last November indulged their ticket-splitting proclivities, Curb became the Republican lieutenant governor while Democrat Brown was winning reelection by a landslide.

This split victory abruptly cast Curb, who takes over as governor whenever Brown ventures beyond the state's borders, as the most important Republican in California. Brown is expected to spend 100 days outside the state this year, first in plumping for a proposed constitutional amendment to balance the federal budget and then in presidential primary campaigning.

So, less than two months in his first public office, Curb already is being measured for the governorship of the nation's most populous state. "Ken Doll"

THERE ARE plenty of Republicans who hope Curb will become the next Ronald Reagan, or at least the next George Murphy. Others, citing the mud-slinging nature of his first campaign, fear he will be the next Richard Nixon. Still others, who find such comparisons far-fetched, worry whether Curb has the substance or savvy to survive in government at all.

"I won't say that Mike isn't heavy, but if he grabs the balloon, he's likely to go up in the air," says one respected veteran of the Reagan administration.

This is close to a pervasive view. Behind his back Curb is called "Ken doll" or "punk rock." On the night of his election the state's Republican chairman, aided by some liquid refreshment, predicted Curb would never be elected to another office.Legislators and reporters who have talked with Curb often suggest that he lacks even civics-book knowledge of the government he may end up running.

For instance, Curb is totally unable to explain what he hopes to accomplish in politics or government. Asked to provide a single candidate, issue or political idea that motivates him, Curb reflectd for several minutes and finally answered, "It's very difficult."

But it may be a mistake to underestimate Mike Curb, a likely occurrence in a state capital which has also underestimated Reagan, Jerry Brown and Howard Jarvis.

Strategically, at least, Curb's idea of raising all the money he could in Hollywood and spending it early on TV advertising paid off in 1978. "The proslooked down their noses at the way Curb ran his campaign," says Assembly Minority Leader Paul Priolo. "But it turned out to be highly effective." Political Opportunity

THE SON of an FBI agent turned corporate lawyer, Curb was born in Savannah and raised in Compton, then an integrated, middle-class Los Angeles suburb that has since become a mostly black community. He started picking out tunes on the piano at age 4 and developed an encyclopedic knowledge of recordings.

Curb says his impetus toward politics came from his Mexican-American grandmother, an Adlai Stevenson Democrat named Eloisa Salazar who died two years ago. The last thing Curb remembers her urging him to do was to run for office. Curb recalls replying that he would go into politics after he became a wealthy businessman.

The political opportunity was provided by Ken Rietz, who in 1972 was chairman of Youth for Nixon in Washington. He asked Curb to provide entertainment for a Miami political rally after Nixon's renomination. Curb came up with Sammy Davis Jr. As some of Curb's friends see it now, the success in Miami convinced Curb that entertainment techniques were readily transferable to politics.

In 1975, Curb was picked by Reagan to manage his California primary campaign, a cut-and-dried affair which Gerald Ford mistakenly tried to turn into a contest. But there was nothing cut-and-dried about the general election after Ford narrowly won the Republican nomination. Curb backed the president and, because he was new to politics and had few enemies, was charged with unifying the Ford and Reagan primary committees. Most Republicans believe this unification helped Ford carry California.

Curb capitalized on his new-found political stature. After discarding the idea of a race for governor, he zeroed in on the No. 2 state post, then held by an embattled black Democrat named Mervyn Dymally.

Curb held a fund-raiser on Dec. 6, 1977, in which, he says, "the entire music industry in Los Angeles turned out." His campaign raised $225,000 in one night, the first stage in a drive for $2.3 million, the most money ever raised in the nation for a lieutenant governor's race. Of this amount, Curb contributed or loaned $400,000 from his own funds and raised $600,000 from entertainment industry sources.

"We were able to create the feeling we could win, which is something many Republicans have a problem doing," Curb says. "And it was enough that the incumbent started to attack us, which is what we wanted."

Dymally's attack proved so inept that it ratified Curb's strategy. Dymally's chief "issue" was that Curb had been involved in a pornographic film called "Mondo Hollywood." The trouble was that the film, of which Curb was musical producer, wasn't pornographic. Long after most Californians realized this, Dymally persisted with the inaccurate charge.

Outspending Dymally by more than 4 to 1 on lavish TV commercials, Curb surged from far behind in the polls to take a clear lead. He almost blew the entire effort just before the election by agreeing publicly with a TV reporter's statement that Dymally was about to be indicted.

Dymally had been questioned, along with others, in an apparently inconclusive FBI probe into official corruption in Sacramento. But he has never been charged with a crime, and Curb's agreement that he would be indicted caused even prominent Republicans to disassociate themselves.

Referring to both the "Mondo Hollywood" and the indictment charges, the Los Angeles Times concluded on the eve of the election: "It is on the basis of such invective and slander that the voters must decide whether Dymally or Curb will stand next in succession to the governorship... For us, the most dispiriting prospect of all is that one of them will win next Tuesday."

Curb was the winner, by 630,000 votes. Soon after his defeat, Dymally retaliated by transferring three commissions that he chaired back to the governor's office and by such petty acts as refusing to hand over the key to the office until the midnight of inauguration.

Curb, learning from his campaign bungle, did not respond to Dymally's actions. Instead, he used them to demonstrate a sense of humor, telling state senators that he would have "an opendoor policy because my office no longer has any doors."

While trying to dispel critics publicly with a quip and a smile, Curb has privately forged a working relationship with Brown. While Brown is in the state, Curb's most important task is to inquire daily about the governor's health. But when Brown is away, as he will be this week, Curb becomes the governor with full powers, including those of appointment.

Both Brown and Curb quickly saw that neither would gain from petty gamesmanship. Accordingly, Curb has promised "a spirit of cooperation" and has praised Brown's budget message as well as his call for a balanced federal budget. "There will be no midnight appointments while the governor is gone," pledges Curb's chief of staff, Sheldon Lyddon.

Brown has promising to work with Curb and has restored to Curb's office two of the commissions Dymally had taken away. As one Brown backer puts it, "There's no reason for Brown and Curb not to cooperate -- they're never going to run against each other." Worrisome Behavior

BROWN, in fact, is running pretty hard for president, and that is fine with Mike Curb. He has put together a staff studded with officials from the Reagan and Ford administrations as well as from the legislature. Many think it could be the nucleus of a good gubernatorial staff if Brown makes it to the White House.

Judged by his staff selections and his delicate negotiations with Brown, Curb would hardly seem the light-weight his critics believe him to be. But there is another side to Curb's early behavior that is more worrisome.

In his first weeks of office, Curb has found the temptation to travel irresistible. He has spoken to party groups throughout the state and has made two eastern trips to GOP meetings. His impact, at least on GOP governors, has been to reinforce the impression of ignorance and insubstantiveness.

"Mike has a tremendous opportunity if he only realizes it," says one Republican who is basically sympathetic to Curb. "He has smarts and guts and good political instincts. But he comes from a culture that never reads or analyzes to one that always does, and he's got to do the hard homework, the book-learning, as well as the handsaking. The only thing he needs to succeed is to realize how little he knows."