Sometimes babies cry. That's just how it is. As long as the parent's doing everything they possibly can, that's all you can do. At that point, I feel worse for the parent than I do for my inconvenience. Personally, I'm much more bothered by my seat back being messed with - kick, hit, tray table abuse, etc.
Just make a plan - are you going to feed her? When? What? Galleys aren't properly equipped for heating up bottles, so if you need help ask when you first get on. Hopefully you've planned for her to be asleep during the flight! Keep a a few things at hand - lovey, new toy, favorite book, a blanket - and stash everything else in the overhead, keeping diapers/clothes accessible.
There are more curmedgeons in first than economy, sorry! They're older, more well-traveled, not used to having families on their side of the curtain. Just ignore them - you and your little one have as much right to be there as them.
Deb