Black Widow vs Karli:
Black Widow should take it. Ultimately, Karli does not come off as especially skilled. She managed in the fight with Bucky and Sam through surprise and numbers, plus Bucky being generally off his game. A one on one fight against Natasha, she doesn't have those advantages. Yes, she's strong and tough, but those don't matter against a much more skilled opponent who has ranged weapons, excellent marksmanship, and enough brains to not get into a melee fight.
As for this alleged overrating, I think the much bigger issue is people presuming that "superhuman must beat human" without any consideration of the actual circumstances of the conflict. For instance, yes, Karli is stronger than Natasha, by a sizable margin. However, strength *doesn't matter* if the opponent never gets into melee range. Its not enough to have a given advantage, it needs to be an advantage you can actually leverage.