OP here, sorry for late response (different time zones!)
I found out that Kate actually RSVP with a maybe. Not sure if that really changes the situation though.
And the wedding was in Kate's home town, both ceremony and reception. A 15 min drive for Kate.
Regarding my friends, who would never burn anyone at the stake , I think they were most puzzled that she didn't perhaps just come for the ceremony.
Once again, thanks for everyone's responses.
I think they need to get over the idea that it's any of their business whether she attended or not. That's between her and the bride.
I don't think the friends have said it was there business, just suprised she didn't attend.
Friend A: I was suprised Kate wasn't at the wedding. Have you talked to her? Is everything ok?
Friend B: I talked to her last week and she said she was feeling fine.
Friend A: Huh, I wonder if something else is going on. Her and bride are so close, I assumed she'd try to be there.
Friend B: I did too, but maybe new motherhood is tougher than we think.
Seems like a standard conversation amongst friends and about friends. The OP only said they were curious if not attending an event like a wedding after giving birth is common. It didn't appear to me they were passing judgement.
Most here have said they think it is very normal for a new mom to not want to attend a formal event. I'm one of the very few who have said that in my experience it would be out of the norm for a new mom to not be able to attend.