Diane is the POV character to Bojack’s world for the first few seasons and she’s arguably the “least worst” of the main cast so some people tend to see her as the “good guy” of the show.

But Diane herself spells out a major theme of the show, that there aren’t inherently good or bad people. Just people who make choices, sometimes they make positive choices and sometimes they make negative choices, and then they have to deal with the consequences.