hearns didn't have a great chin in the sense that it matched most of the rest of his characteristics, but it wasn't bad. he would have never gotten to the levels he reached with a "weak" chin.
he was a massive WW. a stoppage loss to ray leonard is nothing to scoff at. leonard could really punch. it's likely that leonard is one of the only WW in history who would stop hearns. robinson, leonard, and who else really, do you pick to stop tommy hearns at WW?
armstrong? nah. he'd probably lose.
gavilan, napoles, ross? doubt it.
name some other top historic WW. they probably lose to tommy hearns, many via stoppage.
his stoppage losses to leonard and hagler, as well as his long physique and a the tendency of a bodies like that to show damage, contributed to the myth that his chin was as vulnerable as it's made out to be by some.