I have seen people obviously struggling in pretty dreadful situations, post badly written OPs. The sneering often is not just at the spelling its class based as well, a particularly sneery comment I have seen more than once in the time I have been on the forum is :
"We obviously don't move in the same circles OP"
Mumsnet as a Hive Mind both bleats about womens rights and womens opportunities quite heavily but rejects any attempt to engage with women who visibly from posting style were not fortunate enough to receive either a post 16 education or a live in a decent catchment area.
I have also seen women directly facing issues caused by socioeconomic deprivation pulled apart for not fitting a certain lifestyle mould or parenting approach
TLDR : Mumsnet Prejudice is Class Based not Spelling Based