The Vicky Mendoza diagonal is a specific kind of line on the "Hot/Crazy Scale". The Diagonal runs straight from the bottom left corner to the top right, signifying that the woman in question is crazy, but her hotness balances this out.
The Hotter she is the more crazy she is allowed to be.
Also see "Shelly Galesby zone" in which a girl is crazier than she is hot.
Guy1: that girls pretty frickin' crazy.
Guy2: Yeah but shes also hot. definitely on the vicky mendoza diagonal.