Former Playmate of the Year Dani Mathers did a despicable thing -- posting a pic of a naked woman in the gym shower without her permission, and then got shamed herself ... into making a totally empty apology.

Mathers thought it'd be funny to mock the unwitting woman's body -- she captioned the pic, "If I can't unsee this then you can't either."

She got the appropriate backlash as the Internet immediately ripped the 33-23-31 sized model for ... 1. exposing the woman's privacy -- and 2. having the balls to criticize the woman's body.

Playboy's 2015 PMOY then made a video mea culpa, claiming it was an "accident" -- which makes next to no sense.