Nod, smile, agree, thank.
Why are you worrying about how you will respond nastily to people who probably have good intentions and are showing an interest in your baby?
Mostly strangers said things like "what a beautiful baby", "enjoy these days while you can, they don't last long".
There was one occasion when I was in the local shop and dd was poorly and crying constantly. I could tell an elderly lady was building up for a comment and out it came at the till "what's wrong with that baby, it got something wrong with it". Baby was my second and somehow "I don't think so, I'm just a really poor mother" rolled off my tongue. I felt very tired at that time and still felt guilty. TBF dd was a bit of a crier and I can remember someone else saying " can't you keep that baby quiet" and managing to respond "sorry, no, she cries most of the time, aren't you lucky you only have to listen to it here; I have to listen to it all the time. Ed's crying is coming back to me now (she is 17). DS's nursery teacher once announced "that baby's crying again, have you had her checked out". "Yes, she's just a miserable baby".
Literally until she was nearly four unless part I her body was touching part if mine she cried. Amazingly she swanned into nursery and reception and never once looked back.
I'm probably the old buddy in your eyes OP who will beam and say what a lively baby, it's hard work isn't it but you must be doing a good job or if your toddler melts down at the supermarket I might be the lady who asks if you're OK and would you like me to push your trolley so you can carry on.
Try not to make too many enemies - the people you scorn might help you one day.