Are slimmer coaches better coaches? The weight of the evidence says yes. The coaches of six NFL teams the N.Y.Giants, San Diego Chargers, Seattle Seahawks, L.A. Raiders, Philadelphia Eagles, and Washington Redskins spent mealtimes this season sucking down ''thick, delicious'' Ultra Slim-fast shakes. The result: a combined loss of 249 pounds (about the weight of Giant tight end Mark Bavaro) and a combined gain of seven victories over 1989's performance. At season's end, four of the six had made it into the play-offs, and only the Eagles had the worse record than the year before. Each coach received a six-figure fee and a new wardrobe for appearing in the company's ads. Weigh-ins twice a month helped keep the brigade svelte. Philadelphia's Buddy Ryan (minus 34 lbs.) maintained his game frame with an unorthodox version of the diet. ''I mix (the shakes) with a banana and a diet coke,'' he says. Perhaps he should have been stricter. The Redskins, guided by Joe Gibbs (minus 38 lbs.), beat Ryan's Eagles in the first round of the play-offs by a score of 20-6.