Obviously YANBU to be miffed as that is your right. However, as some posters have said, people ALWAYS comment on babies. Whether they are ginger, small, chubby, not walking at one year old, not talking at 3, being very pale, etc etc... So I think you are maybe a tad over sensitive. Especially as most people mean no harm by it.
I do get that constantly hearing it may be a bit annoying though. Don't know why people feel the need to say it to people with mixed race children.
I know someone whose husband gets comments several times a WEEK from May to October, about his very pale skin (he is Irish.. born in the north of England, but has Irish parents who moved there before he was born,) and she gets really upset at the constant 'what's wrong with him, why is he so pale, is he ill, he needs to get a TAN' comments.
So yeah, I get that it may be annoying to hear 'your mixed race baby is going cute etc...' I don't get why it's insulting though. Sorry to be ignorant and I have never had to go through hearing it, but I don't get how 'mixed race babies/people are gorgeous' is an insult. 
I mean 'your pasty faced husband needs a tan, he looks ill' is rude (and she and he hear it 2 dozen times every summer!) but I don't get why 'mixed race people are very attractive' is insulting.
No offence intended. JMHO.