If you read this thread cold then you see that the posters being the most upset about the term are also the ones being the most judgemental about how other women dress.
We all know dads get it too, let's not pretend otherwise, dad jeans, dad dancing, dad bod. And quite frankly the term dad bod is very offensive, can you imagine the outcry if it was mum bod.
And the only place I really see the term mumsy used much is on here. I know no one who uses it in real life, thinks it in real life, and seldom is it used in the mainstream media.
On here though it's a very common term and at least once a week there is a thread of "is this (insert boots, jeans, hairstyle, shoes, dress, top, whatever) too mumsy"
On here it's commonly used and it seems to mean frumpy.