One thing to consider when evaluating Tommy's chin is that at least one of his stoppage losses, the one to Leonard, (I would argue two, throwing in the Hagler fight) came about due to a sustained body assault rather than a problem with his chin. Not surprising given how tall he was for the weights he fought at. This is actually a bit of a bugbear of mine when people argue that Tommy enjoyed huge natural advantages over welter or even middleweight opponents. He didn't. They both had to weigh 147 or 160lbs. So Tommy had to stretch that weight over a longer frame making him less solid than a shorter opponent. So what he gains in reach and height, he loses in physical robustness. Swings and roundabouts. It's why boxing has weight divisions not height divisions. To answer the question, I thought Tommy's chin was fine until he started moving around the weights. He took bombs from Hagler without blinking. Maybe it was the moving around in weight that made him become a bit more vulnerable, as he showed against Andries, Roldan, Kinchen, Leonard 2 (I don't think the Barkley shot showed him to be especially vulnerable. It landed flush. It would have kayoed some rock-jawed fighters). This has been said about Roy, too. Or maybe it was just a bit of age and a bit of wear and tear from the hard fights and the years of sparring in the gym at Kronk. But in his prime he never seemed too troubled by an individual shot to the chin while he was fresh. It was only once his body had been pounded that his chin started to lose some resistance.
I don’t think people are questioning Tommy‘s chin for getting controversially stopped on his feet in the 14th round of a war he was winning against Leonard. At least I don’t. I question his chin because every time Leonard touched it at any stage of the fight Tommy’s legs turned to jelly. Leonard could punch, but he wasn’t Julian Jackson.
I think that's actually the crux of the issue. I don't think his chin was that bad, I think it's his legs. At 147, nobody really had the gall to try and hit his chin before Leonard. I think as he moved up in weight, his legs and base got better but he was facing bigger and better punchers. So he couldn't win. A good example is how he could take Barkley's punch better at 175, but not at 160. Same with Leonard at 168 & 147. Although I guess there is the Kinchen fight which takes away from the argument.