I thought she was the right person for the position, yet it turned out that she was quite useless

Contrary to my initial impression, she was not totally unsuitable for the position.

I was right in my thinking that she was totally useless for the job.

I was mistaken about her suitability for the position, since she proved rather incompetent.

Because I was expecting her to be competent, I was shocked to see her perform rather badly.