When someone possess negative traits to such an extreme that the person appears to exhibit the opposite positive trait.
Similar to how a manatee is so ugly that it is perhaps the most beautiful creature on Earth.
Guy 1: Dude, she's pretty cute.
Guy 2: What are you talking about, she looks like a man, and not a good lookin' one at that.
Guy 1: Oh... I guess you're right, the manatee effect had me going there for a second.