Yes, absolutely toxic a lot of the time.
The vile and vitriol poured by keyboard warriors at people who are obviously upset and vulnerable really winds me up.
The worst was a thread where a 24 year old who was two weeks postpartum and asking for housing advice, ( she and her newborn lived with elderly grandparents with dementia), had pages of posters yelling at her that ( "you're just not normal, three pregnancies by 24 is just disgusting"), despite her stating in the opening post she had lost the previous pregnancy at 17 weeks and PTSD from this).
How anyone felt it acceptable to be so cruel to a young woman who had a two week old newborn, was living with / caring for an elderly grandparent with dementia and was coping with PTSD from the 17 week loss I will never know.
Unfortunately, Mumsnet is imo particularly toxic to: SAHM's, women with accidental pregnancies, young mother's, anyone who has ever had to claim benefits ever, women experiencing DV , ( "you are a disgrace for putting up with this,") anyone with any kind of housing problem or anyone who might be vulnerable.