I know I shouldn't but I'm going to ask.
Why are skinny jeans now on the frumpy pile? I suppose I think of frumpy to be shapeless, unflattering or older than you are.
So in my head, things can't go from being cool to frumpy. Skinny jeans might no longer be the cutting edge of fashion but they can't surely be frumpy if they still look well fitted and you look good in them.
What you actually mean is they are now being worn by people who aren't as cool as you (or rich, or thin) and therefore you need to move on.
The clothes, and you, haven't changed.
The opposite of fashionable or 'on trend' (pokes self in eye with stick) is not frumpy or mumsy.
They are really negative words which says a lot about the people using them.
You set the bar way too high for mere mortals.
To flip it on its head, I would prefer to use stylish vs fashion slave or narcissist.