And perhaps that's the impression he was trying to create.
Playing senile is a good way to avoid tough questions
Did he really think going into this that the Republicans would have him on the run?
No, people like Mueller are cocky and confident, and Mueller lives in a world where people are telling him how wonderful he is.
So if he had all his faculties about him, it's unlikely that he'd be so frightened of the Republicans that he'd pretend to have dementia.
That's not to say that he couldn't have played up his mental problems to avoid answering questions, but I doubt that it was all an act.