There are two important questions:
1) Is she competent? I'd say, in the normal course of her job, yes.
2) Does she have any incentive to do her utmost in questioning Ford? NO. Mitchell is a big city prosecutor. Her job depends on keeping her real bosses happy, and the people her employment depends on will NOT be happy with somebody who was a major factor in favor of Kavanaugh getting on the Court.
She had EVERY incentive to throw the case.
Actually, how she did the job is irrelevant. She was never going to be abe to extract any thing that ws not already known. She was not going to make any difference one way or the other.
The Democraats wanted two things.
To be able to raise a hue and cry of condemnation for hitting the girl
To be able to plant the seed that an FBI investigtion was necessary before the procedural matters could proceed and create from thin air a delay