There's often threads on s&b about what makes something frumpy. People talk about bootcut jeans, fleeces, flat shoes, lank Bob etc. This is probs my definition also.
BUT... there does seem to be a kind of "mum uniform" that is very much not traditional frumpy but could by definition be described as mumsy. So Breton top, skinny jeans, ash jalous or converse shoes. All the fash blogs on here seem to have a similar style... Always skinny jeans, equipment shirts, that type of thing.
Argh, maybe I'm not explaining it well but I wonder if this look is the "new frumpy"? I've dressed like this for years and always felt fairly stylish, but now I'm done having dcs and wanting some new stuff I don't want to end up actually looking mumsy by default.