The media's preoccupation with White House credibility in dealing with President Reagan's recent skin cancer has obscured a more significant and recurrent problem of his presidency.

Reporters properly expect a high standard of truth-telling from public officials, and they are correct in making a fuss when misled. But, while Reagan sometimes confuses facts, his inclination is to deal honestly with the media. Credibility is his long suit, as he showed last week in revealing the truth about his latest cancer surgery.

Reagan's basic problem is not credibility but a frequently superficial understanding of complex issues. While media attention focused on the cancerous pimple removed from Reagan's nose and misleading statements made about this operation, the president again demonstrated that his store of knowledge on the troublesome issue of arms control is often limited to his latest briefing.

Meeting with reporters in the Oval Office, Reagan defended U.S. rejection of a Soviet proposal for a joint moratorium on nuclear testing. Carefully reflecting the briefing he had been given before this session, Reagan pointed out that the Soviets were making this offer because they had completed their recent round of testing. So far, so good.

But Reagan feels impelled to demonstrate that he is well-informed about arms control, and he used his stock answer as a launching pad for a capsule history of past discussions of nuclear moratoriums with the Soviets.

While aides quietly shook their heads, Reagan ended this little lecture by saying: "I would like to add also that after that limited moratorium, which was supposed to end around December or something, if they want to make that a permanent moratorium or if they want to agree with us and have bilateral inspection of each other's testing, we're willing to do that."

The answer did not come close to reflecting administration policy. Only a week earlier, a senior administration official had said that a no-test pledge by the United States and the Soviet Union would "encourage a fraud" since both sides plan to continue testing. Reagan seemed dimly aware of this because, moments after he proposed the mutual moratorium, he referred to the need to test the proposed Midgetman missile, which is years away. In fact, the United States has a diminished interest in an agreement to prevent Soviet nuclear testing because sophisticated, new U.S. equipment can detect even concealed underground blasts.

Privately, administration officials acknowledged that the president had misunderstood his briefing, and they said that no test-ban moratorium is being considered. Publicly, the White House hastily issued a clarification saying Reagan was "not proposing any new initiative" on nuclear testing. When an official was asked what it would take for Reagan actually to propose a moratorium, he responded facetiously, "A mind-expanding drug."

During last year's election campaign, Reagan's advisers launched an intensive and politically successful effort to demonstrate that the president wanted better relations with the Soviets. Reagan has said since the election that he is willing to walk "the extra mile" to obtain a nuclear-arms agreement with the Soviets, and he will hold his first summit with a Soviet leader when he meets Mikhail Gorbachev in Geneva this November.

Continuing the campaign effort, Reagan's aides have worked overtime to persuade reporters that the president is now personally involved in arms-control negotiations, a backhanded suggestion that he was out to lunch on these issues during the first term.

But what Reagan's advisers have failed to do -- and there is no evidence they have tried seriously to do it -- is educate the president on the complexities of nuclear issues so he is at home in discussing them when a question does not match the anticipation of his briefers.

This is a serious shortcoming, and one that does not bode well for the summit. I take Reagan at his word when he says he wants to deal seriously with the Soviets on arms control and other issues. He would have a better chance of accomplishing this purpose if his mastery of the issues matched his intentions.

Reaganism of the Week: Chatting with reporters at his first news conference since June 18, Reagan said: "Well, it's nice to see you all. Where have you been keeping yourselves?"