BAD GIRL MAKES GOOD

''When I heard what the writers had in mind for me,'' moans Melrose Place's newest tenant, Laura Leighton, ''I said, 'Great. America is going to hate me.''' Au contraire. As Sydney Andrews, the anything-but-happy hooker who turns tricks for a Heidi Fleiss-caliber clientele on the Fox soap, Leighton, 25, is fast becoming TV's most beloved tramp. Her sleazy antics this season-which included sleeping with her sister's estranged husband-have garnered the show some of its best ratings ever (one recent episode drew 16 million viewers). More reasons to love/hate Sydney are on the way-including drug addiction. ''She's pretty miserable, and it's going to get worse,'' promises Leighton, who isn't entirely unsympathetic. ''She's insecure and in complete denial. Even though she's a mess, she's trying to take control. And, come on, she had to pay her rent.'' Leighton doesn't worry about her rent now. Though Melrose is her first serious acting job (she had done a few girl-next-door beer and shampoo commercials and was a member of the traveling Young Americans, a perky singing troupe that entertained at halftime shows and conventions), the exposure has radically altered her life. ''Only when you back away do you say, 'Wow, my life has really changed.' It's like hair growing; you don't see it until someone comes up after six months and says, 'Your hair is so long.''' Among the hair- raising changes: a new boyfriend, costar Grant Show, who plays Melrose's resident Easy Rider, Jake. ''We laugh watching each other doing the love scenes (with others),'' she says. Introduced near the end of last season as the irresponsible little sister of Jane (Josie Bissett), Leighton's character was supposed to ''come in for two episodes and annoy (Jane) and leave.'' But now it looks like she'll be sticking around for at least another season-which is fine with her and her family back in her native Iowa. ''They just can't believe I'm on TV at all.'' -Alan Carter