I actually think Sarah is a sweet and thoughtful person. Her weakness is that she allows Jim to make all the decisions (house, decoration etc) and doesn’t seem to assert herself (even physically, she appears slouched and unconfident).
Im sure Jim preys on this vulnerability making her second guess her convictions and become insecure (constant references to his past / harking back to his YouTube glory days).
I don't think Sarah's that sweet or sweet at all. Like when she dropped allegations about Tanya's cheating, pretending she was protecting Margot, and at the same time she kept on plastering her child all over the internet even when Margot was more vulnerable than ever (e.g. when she was poorly lately), exposing her to all the creeps out there so she could buy ugly £200 jumpers. Or when she shaded women who share their kids' progress only because she compares Margot to them and thinks she's not developing as fast as she wants. Or when saying that she is happy to parade Margo so that motherhood wouldn't feel lonely while in reality, it's more for the revenue. I can go on and on.
I agree with someone saying that they're as bad as each other. After all, she was deliberately looking for a partner on a dating app for celebrities. She wanted, and I believe still wants, fame and easy money. That's why she's putting up with all the shiznit Jim does.