Countless times in her career, her mental toughness saved her. Now because she lost where she had a serve to simply tie a game, her mental toughness is limited ? The very fact that she made it a three setter by playing badly all the while Kerber played great is a proof her mental toughness was a factor. Without it, she would have lost in straights, let's be real. Last set of a championship match, she down 0-2 and she ties it. She's then down 2-5 and makes it 4-5 40-30 ? Come on, Serena is the worst example at female mental toughness being limited.
More often than we admit, Serena doesn't play well but she still manage to win thanks to her mental strength. What was the last major match she actually played great ? Last year, she had numerous 3 setter wins where she was absolutely awful though she still overcame. Her French Open run was quite a struggle for example.