Emilie you haven't been an awful unsupportive friend, you're coping with a situation that would test everyone. My friend has a beautiful ds with spina bifida and he couldn't be more loved, but it is obviously very challenging.
I agree people with no fertility issues and children with no disabilities can really take it for granted and it's difficult to tolerate sometimes when people say insensitive things. I don't take my dc's health for granted as I have a severely autistic sister, which is heritable so I fully accept we could have a child with this condition.
Don't feel bad, it's not your job to be a good friend, a good friend would understand what you're going through even if they don't know about your mc.
To answer the op question, you just have to smile and say congratulations but allow yourself to be upset after. I avoid seeing friends who constantly ask me when I'm having dc2, I tolerate being asked once but those that bring it up everytime I see them have been dropped. I have asked dh not to tell me about ppl who are pregnant unless it would be weird for me to not know.
I only have one pregnant friend at the moment who luckily does not go on about it. When I see her I ask her about her pregnancy right at the end of the meet up 'oh sorry I'd forgotten you were pregnant you look so slim blah blah' then I know it's not going to dominate the meet up.
But I'm lucky it's not a family member or someone I can't avoid.