Katara doesn't fight for evil, but if she's mad or protective, Amon better watch his ass. He doesn't have any emotion to his fighting, just brutality and force. If Katara was in grizzly bear mother mode, there would be no chance she'd lose.
Amon would win. He broke out of Tarrlok's grip and took his bending away. Unless Aang was there, Katara wouldn't stand a chance. Scratch that, even if Aang was there, Katara wouldn't stand a chance. Amon is far more powerful than Yakoan, and would beat Aang just as easily as he beat Korra.