Some blokes are tossers, some blokes are brilliant. Some blokes are abusive narcissists, some aren't.
The exact same with women. It's just a fact of life.
People (women, men, whatever) rarely complain when there's nothing wrong.
Posters come to the forums when they're in need of help/assistance, therefore it's reasonable that we see more men who are a waste of space than those who aren't.
No one regularly posts threads being like 'oh my husband's great'. I think a lot gets dramatised on social media and can often turn into a pissing contest of 'oh your husband does this well MINE does THIS!'
What you see on Mumsnet I would hazard a guess is in no way reflective of real life in any sense. See Mumsnet Chicken, Parking Wars, Dramatic Interludes etc. Threads blow up for the drama, people love it when they're not involved. Hence why we see so many 'waste of space' examples.
I have been lucky enough that none of the men in my immediate life are wastes of space, however I know far too many friends whose husbands/fathers/brothers/sons are. The same way I know lots whose mothers/wives/daughters/sisters are.
I'd say women are quite well reflected on this platform, in comparison. I've known far more toxic/abusive women than I have men. However, if we were to hold Mumsnet as gospel that wouldn't be the case.