Momma Beaumont may do everything perfectly (it's her savvy), but she is definitely not perfect. She even says so herself:
"I'm not perfect, Mibs. Nobody's perfect. I just have a knack for getting things right. Maybe that looks a lot like perfect sometimes." (11.21)
What this means is that though it may seem like she's perfect to outsiders, Momma knows in her heart of hearts that she's just as flawed as everybody else. In this way, Momma is part of the general message in this book: don't judge a book by its cover.
Momma does her best to prepare her kids for the world that they have to live in, even though she can't always be there for them. She leaves Fish, Mibs, Samson, and Gypsy by themselves when she goes to tend to Poppa in the hospital (does that seem weird to you?) and leaves Mibs alone on her most important birthday. And though it isn't exactly a character flaw to not always be able to be with your kids, that Momma can't be definitely puts a crack in her perfect façade.
Momma tries her best though, and her family really loves her. She's the perfect mom, the perfect wife, and perfectly lovely, so how could you not adore her? Though life isn't perfect, Momma deals with the pitfalls that are thrown in front of her family just as she's supposed to: in the just the right way.