I really don't think that people should compare two different kinds of loss/suffering as if there's a top trumps of being upset.
Loving a person and being very sad at their death is painful and there are reasons why grief counselling exists - it can take years for someone to come to terms with losing a parent they loved.
Growing up with shitty parents who effectively steal your childhood from you and leave you damaged for life is also hard to recover from (and some people never do), which, again, is why specialist counselling exists.
It is a shame that your friends couldn't see that both you and the bereaved friend are suffering, although for different reasons, and that mothers' day will be particularly difficult for each of you. As they're so concerned about your friend, it makes no sense that they posted in the first place, as surely drawing attention to their living, happy relationships is even worse?
You should be able to post 'fuck all', and she should be able to post about her feelings, without either one of you feeling hard done by. You aren't trying to say she shouldn't feel grief, or pretend that she didn't love her mother because of your situation, and you shouldn't have to modify your emotional response either.
It seems like we all have to pretend that a saint gave birth to us on mothers' day, and for those of us who had a different experience with parents, we're not allowed to voice our hurt.
Perhaps let the dust settle, or send the bereaved friend a personal message, but it sounds like your group of friends having really understood how hard growing up with bad parents can be.