Mostly what Djembe said although I don't agree about the weight thing tbh. We all know lots and lots of women who are larger than a size 12 who aren't Mumsy. I think that may be the OP's interpretation rather than general S&B opinion. I went from a 16 to a 12 recently, I wasn't Mumsy before at a 16 but I have found that there are more options available to me now I'm a 12, I wouldn't have worn skinny jeans before but I do now for example.
I don't think Mumsy is a generic term to mean like a mother, that would be far too sweeping. I think of it as a look that says 'I'm not bothered, I'm a mother now'. I was one of the first of my friends to have children and I've been amazed how many women who were once well presented and or stylish who simply give up once they have children. I had three dc's in three years and of course I had many a day where I barely got out of my PJ's but overall I managed to spend five minutes putting on some lippy and run a brush through my hair. Bootcuts and tunics are deemed Mumsy as they are not just not in fashion, they are very much out of fashion so to be wearing them implies that you have totally given up on keeping up to date.
You may say who cares, fashion is facile and nonsense but I enjoy it, it makes me feel great, I feel more confident and I like to keep up to date on all sorts of modern culture, music, design, politics and technolody so for me to be walking around in bootcuts and a tunic would be at odds with the person I am. If I were to be accused of being sneery (which I hold my hand up to) it's because it feels so old fashioned and of our mothers generation to give up on all things contemporary because you've got to a certain age or become a mother. In much the same way I would if my DH started smoking a pipe and calling me mother. I think that if you believe that women can have it all then they can continue to look good too.