LIZA MINNELLI has rarely looked or seemed healthier these days. She's everywhere, like a fly on a griddle - concertizing, picking up awards, attending the theater - she adored the current "Jacques Brel" revival. At L.A.'s House of Blues last week, she attended the 14th annual American Society of Young Musicians event where the youngsters mobbed her. She's working on a movie script about nightclub life in N.Y.C. - something she used to know a lot about! - and will soon record a tribute album honoring her late godmother, author and dazzling performer Kay Thompson. (A lot of Thompson's dramatic style was picked up by Liza's mom, Judy Garland, who honed it to perfection.) Now, folks, isn't it time for Miss Minnelli to collect the ultimate American honor from the Kennedy Center? I say yes!