Certainly, people have said things such as " MN is transphobic", but people say things like " Tunbridge Wells is all right wing Tories or Ukip". It is never an accurate picture, that sort of thing.
Some TS or other people have posted things on MN and got some pretty uncompromising and not very welcoming responses, tiz true. Sometimes those responses seemed to have less to do with what the poster was saying, or standing for, and more to do with the poster's underlying attitudes towards " people like them". Or " people like them " being on mumsnet, even.
But it can never be the full picture of a thing as big as MN , and there have always been posts that take a less hostile attitude. There are also occasional pms which say encouraging things.
As for numbers on MN who fall into various categories of opinion about transgender people or the smaller subdivision, transsexual people , who knows?