[I understand where you are coming from, but I think it's important to understand not everyone thinks or feels the same way. For me, positivity and looking on the bright side is what I need when I am hurting. If this happened to me, I would imagine I would be grounded and calmed focusing on what I did have (a baby who would be healthy and coming home) rather than what I had lost, which would likely panic me. Obviously not everyone agrees.
For others, like you mom, positive comments apparently upset her. It doesn't mean your mom is right and I am wrong, or vice versa, just that people are different. To let someone "have it" when they are trying to be supportive and are likely saying what they would want to hear if the positions were reversed seems counterproductive. To gently explain what you need instead would make sense, depending on the relationship.
I agree that everyone manages hard situations in different ways. I would like to point out though, that you really can't say how you'd be until you go through it yourself (this is true for anything). I too, like to look at the bright side of things and be positive. And like I said before, normally people pointing out the good things makes me feel better. But, take into account that you (general) are speaking to someone who probably isn't their normal, rational self. Post partum, I was emotional, irrational, and illogical. And I haven't talked to a single mother who has had a baby in the NICU that wasn't the same way. Sure, there may be someone there that would love to hear the "at least the baby isn't really sick" comment, but I would err on the side of the majority and think that it probably wouldn't be helpful.
And I think telling a mother in this kind of mental state of heartache and stress not to take it personally, won't help at the time. Logically, she probably already knows this, but emotionally she may not be able to get herself there.
In a situation where I'm trying to comfort someone who is going through this, I would likely focus on what the mom is feeling and try to be understanding that she is going through something rough...even if things are guaranteed to get better. And I would do this until I know, for a fact, that she wants me say the "well, at least you don't have it as bad as those whose kids are really sick" statements.
I think Turtledove's point is that this approach won't be comforting to everyone in the situation. I've been there. DS was born 5 weeks early and was in NICU. In the begining his chances of surviving were remote. My emotional state would have been much worse if the people around me had been trying to assuage my feelings. Instead I needed assurances that DS would get better, that there would be no lasting damage, and that his situation would, in the end, make us all stronger. Fortunately, I got what I needed. ..mostly.
My point is that everyone handles stress differently and what is reassuring to me might be insensitive to you. Since we can't really know where someone else is coming from, it's best to take their efforts at comfort in the best possible light.
What you are describing is a very different scenario than, I think, the one we are debating. Your baby was sick. I can't even imagine what I would say to a mother who was feeling the stress of wondering if her baby would survive. In that case, I actually have no idea if it's better to throw out "everything will be ok" or focus on the parents' stress.
I'm talking about the situation in the OP, and the situation I was in with my own baby in the NICU after she was born 5 1/2 weeks early...and that is, having someone tell me "at least she's not sick" and "you can bring her home soon enough" were not helpful. And since I was in the part of the NICU with other babies in similar situations to my DD (i.e. there to gain weight, get jaundice treatment, or for a mandatory stay because of prematurity and not actually in a life threatening situation), my experience is that most mothers felt as I did. Extreme stress and sadness over not getting to bring baby home and the feeling of separation from the child. All of us would have appreciated "how are you doing?" and "this must be so hard to leave your baby in the hospital" over having our feelings minimized.
My point is simple. Of course people react to things differently, but I have never met someone who got offended at a "how are you doing?" question. Maybe she says "I'm a mess, I just need someone to tell me it's going to be ok" or maybe she says "I'm a mess, rationally, I know everything is fine, but I miss my baby so much, I feel so alone and empty." Then take your cues from her. Jumping in with positives does sort of sound like you're (general) minimizing her feelings. She doesn't have to have a sick baby to feel upset about being in the NICU.