Neither the public nor US leaders accept President Reagan's effort to increase arms spending or his belief in Soviet military superiority. That's the verdict of an article in the current Foreign Policy magazine, based on a Gallup poll. The article was written by John Rielly, an adviser to Walter Mondale, who is seeking the Democratic presidential nomination. The study showed many more Americans favor higher spending for education (59 percent), social security (49 percent), and welfare and relief (27 percent), than for defense (24 percent). Nearly two of three Americans believe the US is militarily as strong as the Soviet Union or stronger.