Much like MN, I find people turn into armchair experts when they spot something that needs "correcting". It is because your child is very young that people don't see it as offensive because they know the child will grow up and out of their current looks.
Personally I would - politely obv ;) - make a comment back that puts them on the backfoot...
"Thank you for noticing, I noticed that your dress isn't all that flattering, it clings on the butt too much"
"I am SO glad you told me! I didn't even notice! That is new information!"
"Didn't you get the memo that its rude to point out the flaws in other people?" [absolutely marvellous in a group, it is an instant "ouch" moment, use with extreme caution though! Only on people you aren't all that close with]
"Finally I have someone who can perform surgery on my poor baby's eye. When can you slot us in!"
"Well at least he can look at you and for you at the same time!"
"Yes we call him the mince pie (lazy-eye)"
Anything like that will almost always shut people up or at least create that wonderful "oh this is awkward" moment. I use this type of approach to people who I am less bothered about (offending) or when I know someone so well that they think "okay, yeah I was a bit out of order there". The key is to approach it with such sincerity, a big smile, open posture and real interest. It should feel like it comes from a place of compassion, not sarcasm. You want to give them the message in such a way where they notice the absurdity of their comment. I am an extremely irreverent person at the best of times, so I can get away with it a bit more. Maybe try it, though don't go too hard early on as people tend to get a tad discomforted.