The phrase “mutton dressed as lamb” comes from the practice of butchers or restaurants passing off the meat of older sheep (mutton, which was cheaper and an everyday meat) as lamb (which was more expensive and eaten more rarely). Back when mutton was a cheap meat eaten very widely in stews, etc.
The use of the metaphor to apply to women might be sexist and outdated, but it in itself is rather creative and descriptive (think of the old-fashioned habit of serving lamb ribs with paper frills on - that’s the “dressing” of the meat!) And it’s still common that lots of meat sold in some stews and curries as “lamb” (for example in ready meals) is actually technically mutton. (Mutton actually takes the flavour in stews and curries better.)
Anyway, OP, I’m 46 and I have to say I probably would have worn it when I was 20s, but not now - I’d probably have thought it was a bit young for me in style in my early 30s to be honest - not for any ageist reason, but for the fact that I think ruffles and frills are intrinsically a bit silly and meant to look like a stereotypical idea of little girls’ clothes. Yes women should wear what they want, but isn’t it also a bit sexist for women’s clothes to be designed to look like little girls’ dresses? (those flouncy long dresses upthread are awful btw!) Not that little girls have to dress like that either! It’s a nice dress, but isn’t it intrinsically a bit weird for a dress for adult women to look “cute”?