Why aren’t we surprised that Britney Spears wears mascara and lipstick to the gym? The normally blond pop tart, who also sported unbecoming jet-black hair, was on a treadmill and doing crunches at the Mondrian Hotel in Hollywood the other day with her mother, who used dumbbells. Britney worked up such a sweat, “she looked like Heath Ledger as the Joker,” said a witness. “But she has a great butt.” Celebrity publicists Adam Weiss and Todd Shapiro told her, “You’re an inspiration” — to which she scoffed, “Yeah, right!”