Carol Monaghan is Connie's mother and the Berglunds' next-door neighbor in St. Paul. After the mostly flattering portrait we receive of Patty very early in the book, it's through Connie's sneering gossip about Patty that we begin to peek behind her fake smile.
For most of the book, Carol remains a character who mainly helps us learn more about other ones. For example, it's through her phone call to Joey that we first hear about Connie's increasing depression. Of course, this is pretty much exclusively tied to Joey treating Connie worse and worse during his time at college. So this call becomes our best evidence of the pain that Joey selfishly causes the people around him. When he was a teenager, Carol thought Joey was the bee's knees (and, truth be told, has sort of a creepy crush on him). After welcoming him into her house and treating him as family, he has no qualms turning his back on her when he doesn't need anything else from her.
But we've already covered Joey. What's important here is that Carol, much to her dismay, ends up aligned with her archrival Patty. She tells Joey on the phone, "I never thought I'd hear myself saying this, but I'm starting to get an idea of what you put your mom through. Seriously. I never understood how cold you are until this fall" (3.2.47). We might've still wanted to root for Joey despite hearing his mother's side of things – and, hey, we still might root for him – but this is pretty harsh.