To be brutally honest, I think that some mental conditions should negate one's "right" to have a child. I don't mean it in a bad way, but some people before have said it perfectly - If they cannot look after themselves, how can they look after a child? We're not saying "He has Downs, ergo no children for him", we're using simple logic. A child requires a lot of care, attention and money, and if someone is permanently in the care of another person (Such as someone with a serious mental handicap) and requires that same amount of attention, then it's not fair on anyone involved.
But you could say the same for people with physical handicaps too. I knew a girl in one of my classes and she had a condition (I don't know what it was) that made her almost unable to move. Her joints were mostly locked in place and whilst she could stand with support, she wasn't able to walk or really move. If she was able to procreate (I don't know if she was, but I kinda doubt it), there's possibly no way she'd be able to complete her pregnancy and be able to care for that child properly.
I think, though, a bigger problem is people having children who don't deserve them. Instead of breeding like rabbits, I think people should be more responsible and foster or adopt a child. Instead of adding to the world's population, make (at least) one child happy by giving them a chance at a proper life.