Please note that Margo thinks evil incarnate is the "psychic" grifter who tried to overcharge her mother on a wedding venue. Not the former assistant who tried to explode a bomb wrapped in graph paper at her apartment.
I mean "evil incarnate" is reserved for like Hitler or Charles Manson. Diane is just your average flim flam artist, trying to make a buck or several hundred thousand by exploiting extremely gullible, wealthy older brides. I'd say that rates a "barely menacing" on the evil scale.