No!
In cricketing terms he's well behind Graeme "balls-of-steel" Smith, Shiv "unnatural powers of concentration" Chanderpaul, Mohammed "scrapper" Yousuf with regards to mentality. If you're referring to overcoming adversity with regards to leaving cricket South Africa, he's not done anything a lot of other Kolpaks have done. Besides, he's been far more privileged than a lot of other cricketers- think what Makhaya Ntini the cattle farmer had to overcome to achieve his dream of international cricket.
But back to what matters, the cricket, for me he's someone who scores an awful lot of his runs in the first innings of matches when the pressure is off- he scores against excellent attacks, yes, but that is more testament to his ability than mentality. Indeed, he seems to have trouble motivating himself so strongly against teams that aren't South Africa or Australia. If I was picking a team of international cricketers to bat for two days to save a match, Pietersen would be nowhere near the side. He'd probably hang his bat out to a Jacques Kallis 75mpher outside off and get out for 15 off 4.
Not that I'm not a fan of the man, I think he's a little over-rated, yes, but he's extremely talented. All I'm saying is, mental toughness isn't his strength. If he was, as you suggest, as mentally tough as he is a talented cricketer, he'd surely be averaging somewhere in the 70s.