This narrative is far from explicit, I don't think anyone will actually say women are free from prejudice but in reality it is implied to be so. After the 2016 American elections, so many people were shocked that a large percentage of Trump voters were white women because women are assumed to be more left-leaning.
On the George Floyd thread, some posters made remarks like, "It's just male violence" ignoring the fact that female police officers in the US can also be racist which leads to police brutality. On the thread about the wives of the neo-nazis "protecting" the cenotaph, many posters are implying that the wives are being abused or are to ignorant to realise their husbands are terrible, ignoring the obvious reason that they too are probably neo- nazis as well.
As I said, it's not just on mumsnet, it's also in popular media. The controversial movie "Green Book", portrays the wife as a reluctant racist. She is never explicitly racist, like her husband, and seems to just go along with it to keep the peace.
There is even this undercurrent that if women are racist, it's far less harmful compared to men being racist. Racism from women is seen as "racism-light" which makes sense as women are generally unlikely to be in positions of power. However, this idea no longer applies in 2020 when more and more women are finally becoming more equal in society.
I haven't experienced this narrative that "white women are the worst" or they are "punching bags" for everyone, white women are not n-word of the world, that title belongs to black people. I also believe all non-black people harbour unconscious anti-black prejudice, and some black people can even internalise this. So I'm not saying only white women are racist ( I didn't include white in my post).
I do accept that women are held to a higher standard than men when it comes to social justice issues because racism often comes with sexism so a woman voting in a racist is voting against her own interests imo.