I would normally be on your side, but
1) It isn't 'one' bad performance.
2) Regardless of confidence and wages, a PL team should be at least having shots and dominating the game. From the sounds of it we did neither.
3) The tactics were completely shite.
There is little to suggest an overreaction here tbh.
Yes when you look at the home performances overall this season this is no surprise and Lambert is hoping for things to change rather than trying to make things happen. For instance only one sub today when clearly we were being outplayed. The FA Cup may not mean a lot to some people but it was our only chance of silverware.