Serena had almost 20 unforced errors in the final set. Including the match point.
She had 18 unforced errors to be accurate. In the first set against Sharapova, she had 16 unforced errors. How in doubt did that set seem?
There are unforced errors and there are unforced errors. First, it's the nature of the beast that in tight matches power hitters are almost always going to have more unforced errors than counterpunchers/retrievers. Power hitters go for more because they want to end points; counterpunchers go for less because they want to keep the ball in play. That fact accounts for a lot of the errors that you saw in the third set last night.
Point two, not all unforced errors are created equal. Plopping a soft second serve into the net when you are under pressure is one kind of error; catching the tape on a relatively easy shot at the end of a long, exhausting rally is another type of error. In the third set, Serena had far more of the latter than of the former.
Ask yourself this: look at Serena's play in the third set in its entirety. Given that level of play, do you actually think she would have lost if she were playing anybody else, Sharapova, Radwanska, Muguruza, et al? That third set was not just great theatre, it was great tennis. Serena didn't give Kerber the championship, Kerber won it by defeating Serena.