I don't think the issue was the actor - just think Karly and John Walker had inconsistencies in their characters that performances couldn't cover for. The way their characters 'escalated' didn't track to me - the wilful murder of the soldiers in Karly's case, and the deathmatch with Sam and Bucky in John's. All too sudden.
I suspect that was down to the show being too compressed, rushed (definitely think the story change took a toll) and some bonkers editing (so much that was shown should have been on the cutting room floor). I think in a better version of the show, the actor playing Karly would have been spot on. I think the writers an showrunners let her down.