'Biggest Loser' Trainer RIPS Jillian Michaels -- She EMBARRASSED Herself at Malibu Triathlon

Jillian Michaels didn't just SUCK ASS at the Malibu Triathlon -- she was an EMBARRASSMENT -- so says her former "Biggest Loser" co-star Cara Castronuova.

As we previously reported, Michaels was a disaster at last weekend's big race -- admitting to TMZ she got beat by a bunch of people a world class fitness professional shouldn't have lost to ... including an 80-year-old.

But when we ran into Cara last night in L.A., the former B.L. trainer didn't pull punches -- telling us. "I would probably kill myself if that happened to me."

Cara said Michaels likely bombed because she didn't take the event seriously -- "That's what happens a lot with people that think they're in really good shape. They take things lightly and then they embarrass themselves."