Many people say Frazier had a weaker jaw than Ali but I believe they had an equal amount of strength...
They both had good chins. Ali didn't have to take much punishment his first reign as Champion. He didn't really start to absorb punches until he came back from exile. The difference in my view is that Frazier had to walk into his opponents to be effective. He took more clean shots even though he was bobbing and weaving. Ali had another dimension where he could use his size to protect himself from further clean punches when he was stung. They both took a helluva punch.
Frazier technical flaw was he would bob and move his head but not close the distance. So Ali may miss punches one and two but land three and four. Joe also had the habit of picking his head up out of his crouch after dodging a few punches. Ali would tag him constantly with a follow up hook.