In the back garden of his favorite Chinese herb shop in West Hollywood, Brendan Fraser is---BOOM-boom- BOOM-BOOM-BOOM- boom-boom---doing the upright-bass version of  the George of the Jungle theme, and leaning over his apricot tea to pick invisible nits off your clothing.  Where a moment before he was just Brendan---sweet, soft-spoken, seemingly oblivious to his own sizable animal magnetism---now he's George, idiot monkey-boy.    He gets off his chair and starts cruising around the table in a kind of semiseated crouch---back straight, fingertips nearly touching the ground---interrupting a gardenful of ginseng junkies to make the point that he truly enjoyed playing the twisted Tarzan in Disneys' live-action version of the late-'60s spoof cartoon.  Thought this 28-year-old actor, in his more reflective moments would like to be thought of as an artiste---saying, for example, that his thawed-out Neanderthal in Encino Man was his "homage to silent film acting"---he's at his best when he's being playful.  He pulls his T-shirt down from his shoulder and says, about one of the scars he imagined he got on George, "That's the Tookie-Tookie bird right there"; and then, holding out his large, handsome hand, "The sloth bit my knuckle."    When asked how it was to be practically naked through the whole shoot, he says, "Cold," and laughs at his own joke so hard that he has to cover his face in embarrassment.  When asked if the set was heated, he says, "Yes, but it was still cold: They had to open the doors to let the elephant in and out," and dies laughing.        Fraser, it turns out, was happy to be almost naked for part of George: He couldn't play the character, he says, until he had his costume---his body, that is---blown up to cartoon size, via four months of arduous weight training.       "It got to the point where I had to turn sideways to get through passages in the set," he says.  "They had to saw a hole for me to get out to the bathroom."      Finally, he sums it up---his satisfaction with playing George.  "I am," he says, "comfortable in my own skin."               Trish Deitch Rohrer