The "Mirro Dress" for "fatso figures" was one of a number of unusual items that Kaufmann's Department Store included in an ad that it ran in the Pittsburgh Post-Gazette on April 1, 1949
. Other items included a "Sun-Tan Umbrella" that browned you with ultraviolet rays as you walked in the rain, and "Grow Cup" ceramic paste that could regrow handles on broken coffee cups.
The ad was an April Fool's Day spoof. Nowadays spoof ads are a dime a dozen on April 1st, but back in the 1940s they were nonexistent — except for this one. In fact, this is the earliest April 1st spoof ad that I'm aware of.
The practice of creating April Fool spoof ads only really took off in the 1980s, following the success of the Guardian's 1977 San Serriffe hoax
. And it was only during the past decade that just about every company you could imagine jumped on the spoof ad bandwagon, resulting in the present situation, which is a flood of spoof ads every April 1st.