That's because people don't understand it. Yes, it can refer to a woman inappropriately dressed for her age, which is ageist and misogynist, but it can also refer to anything which is passing itself off as something else. The term 'dressed' in the phrase refers to 'dressing' meat, i.e. preparing and cooking it, the point being that mutton (which is sheep meat from wethers, that's overwintered lambs) is cooked and presented in a different way from spring lamb, which is tenderer and more forgiving of cooking method (tho' lamb generally is very forgiving as long as you don't incinerate it), and if you use lamb cookery on mutton it won't taste very good. One could say that most Victorian architecture is mutton dressed as lamb if you don't like Victorian gothic (I do like it, tho' the Victorians could not be trusted with anything actually Medieval since they saw most pre 1485 architecture as not properly gothic!). Context matters, of course, and OP used it (and I replied) with the misogynistic sense - she chose it, not me, but we all instantly knew what she meant. Would it be any better for her to say is this too old for me at 60, or will I look as if I'm trying to be twenty again if I wear this? They are also misogynistic and ageist if one is going to criticise clothing on the basis of age.