Question: What's the quickest way to turn hot celebs into cold stiffs? Answer: Make dolls out of them. Expected to generate big sales this Christmas, celebrity effigies brought little cheer to many of their real-life models. Matchbox's Cheryl Tiegs, Christie Brinkley, and Beverly Johnson dolls were badly upstaged by Barbie, who did an estimated $700 million in sales in 1990. Johnson insists that having a lifelike plastic image of yourself still has its advantages. Now, she says, ''I don't have to worry about wrinkles.'' On the male side of dolldom, Warren Beatty won't be cashing in on Steve the Tramp, one of the Playmate-Disney line of Dick Tracy dolls, based on a character in Beatty's summer fantasy. The doll, described on the packaging as a Fagin-like mistreater of children, sparked a bitter pre-Christmas protest by the Coalition for the Homeless. The group claimed the doll promoted a negative stereotype of homeless people, and Disney pulled it from the market. The season's most successful celeb dolls were based on pop icons who are themselves manufactured. Atop the Toy Manufacturers of America's 1990 best- selling toys list were Mattel's Simpsons dolls and Playmates' Teenage Mutant Ninja Turtles.