It's funny, but it's almost certainly not a picture of an actual sign. It's probably one of many digitally altered "real" signs. If an employee did manage to get that up on a real sign, it wouldn't have been up for very long. The last thing a national restaurant chain is likely to do is to take a politically partisan stance and alienate half the American people. Another thing they are not likely to do, especially Kentucky Fried Chicken, is to use the word "fat" in describing a menu item. Nor are they likely to insult women with a "fat thighs" joke. If this were real, you can bet CNN would be all over it, and KFC's survival would be at stake.
Plus, I've seen the same exact picture with a Tonya Hardin Special, 2 Small Breasts, 2 Large Thighs, and a Club Sandwich.