Not that we're merchants of gloom, the latest figures for magazine advertising are dismal. Tallies of the number of pages carrying advertising in the first quarter, an early indicator of publishing woes, are down by double-digit percentages at news weeklies such as Time and business magazines such as Business Week. The only surprise is that Keith Kelly, who published the figures in today's Post, didn't tweak Mort Zuckerman, proprietor of a rival tabloid. Zuckerman's pet news magazine, US News & World Report, fell 37.5%. One perky spot: Martha Stewart's Everyday Food, now the last best hope of the magazine industry, as well as frustrated cookie-bakers.