The Cat in the Hat is back as a fashion upstart. The kooky, squashed tramp lid Dr. Seuss bestowed upon his incorrigible feline more than 30 years ago seems as ubiquitous as tattoos among the MTV set. Veejay Julie Brown, outfitted by New York boutique Yoshi, introduced the look on the air eight months ago, and at the Grammy awards in February, Seuss hats rivaled seamed stockings for Most Groovy Accessory. L.L. Cool J is so smitten he has over 25 floppies in red- and-black velvet and leather. The Black Crowes' Chris Robinson swaggers about in a Rastafarian interpretation striped in red, yellow, black, and green leather; Bobby Brown, Lisa Stansfield, and Lenny Kravitz also wear the ''floppy topper'' or ''floppy turn-up,'' as the whimsical stovepipe is variously known. The trend is reaching beyond the musical set too. thirtysomething's Nancy Weston (Patricia Wettig) recovers from chemotherapy in a black velvet topper. Looking very Annie Hall, Diane Keaton pulls one down over her eyes on the cover of Mirabella; Jamie Lee Curtis, in orange scrunch hat and nose ring, poses as a trend casualty on the cover of Spy. Whence the stylish high-top? Up, apparently. Paris hatmaker Jacques Le Corre began concocting the hats four years ago from such floppable fabrics as rayon, linen, and cotton, but until recently his cockeyed creations were available only in select small shops in New York and L.A. Now Bergdorf Goodman's millinery department swims in spring Seuss hats at $225 each. ''It is an important trend, very current,'' says a reverent spokesperson for the store, and cheap knockoffs are cropping up on streetwise neohippies. The sloppy floppy is not for everyone, though (Roseanne Barr, Phil Collins, Carnie Wilson, take note). As the Big Cat himself said: ''It is fun to have fun, but you have to know how.''