By Jim Farber
Updated September 24, 1999 at 04:00 AM EDT

Christina Aguilera had better watch her back. Simpson not only sounds like an adenoidal version of Mariah Carey, she’s got the leggy looks of a Dallas Cowboys cheerleader. It’s small wonder Simpson comes to the world via Tommy Mottola, who discovered the 19-year-old on the Christian music circuit. Just don’t expect to hear a whiff of gospel in her ballad. ”Lover” is so conformist, it makes Diane Warren’s stuff sound eccentric. C-