Karen had a succession of plotlines that spotlighted the fractured relationship between her and Diana. Most of the other ladies did not have that, and I guess it makes them look like "better" mothers simply because we didn't see the mother/child relationship examined quite as closely. Much of their mothering was happening off-screen. Abby and Olivia were already somewhat distant due to the secret about the twins when Olivia started taking drugs. After Abby took drastic action to get Olivia off the drugs, Olivia got back to normal for the most part. In both cases there was an outside source driving them apart (Abby's secret and Olivia's addiction) and once it was removed, they went back to a more typical relationship. Karen and Diana had no outside source causing their problems; they just were very much alike and pushed one another's buttons. That sort of problem can't be overcome as easily or neatly.