To review, in fashion, "flats" are the technical sketches designers make in Adobe Illustrator as sort of diagrams for what they're trying to get the factory to make. "Specs" are the measurements given as specifications for the dimensions of the garments.
Annnnd now, I'm going to ruin the funniness of the joke by explaining even further why it's funny. The sketches on on the mug are actually flat sketches of accessories. Get it? Flats of flats? Specs of Specs!?
Yep. I love puns. I think most spinsters probably do. Check it out at the zazzle spinster store. And groan at how bad my joke is.