My first encounter with this expression was in the seventies. A biker chic was wearing a shirt with the slogan. Beneath the words a pointed finger pointed to the subject referenced, her big, tough, mean looking biker better half was sporting the counterpart T-shirt, identifying himself as the "stupid" she was with. It was a playfull, relatable indication that one shouldn't assume things from outward appearances. This guy might look mean but he was probably a big softy. He was comfortable enough with who he was to tell the world through their coordinated ensembles that he shared the power in their relationship.
Later, I would see couples where the sexes were reversed. There would be an unhappy looking woman wearing the "stupid" shirt, with a cocky husband strutting at her side, pointing her out as if thought that through verbal the subjugation of his wife the world would see him as being quite the man.
The statement itself takes on meaning depending upon the usage. I don't think it's an endorsement, per say.
Best Answer
The expression appears to be from the mid/late '70s. One of the earliest usage instances I could find is From Ann Lander's Encyclopedia, A to Z , 1979 and it refers to a T-shirt usage: