You only have to look at Haglers record in his defenses to see the gap in class. Hagler fought in an era of fantastic middleweight boxers and was undefeated from something like mid 70s until the highly questionable split decision loss to Leonard in 1987.
His 2 losses and 2 draws were pretty much dodgy home town decisions when he fought on the road, the Leonard decision just being bad period.
The guy was middleweight champion for a crazy amount of time...but agin look at the guys he fought...and he was a top contender who wasn't given a title shot for years cause he was so feared.
He beat John the beast Mugabi who was 26-0 with 26 kos! He beat Tommy the hitman Hearns in his prime, he beat Roberto Duran! and he did beat Sugar Ray Leonard no matter what those idiot judges said.
I could mention loads more decent fighters....who did Hopkins beat in comparison?
Hopkins was beaten by Roy Jones in his first big middleweight shot, he was beaten TWICE by Jermaine Taylor who has been shown to have been much more limited than people first thought, he has been beaten by Calzaghe albeit at a heavierweight class (has some bearing in terms of legacy) etc....
The records are not even close to being comprable IMHO.
It seems to me that it is just one of those situations where it is easy to forget quite how great Hagler was in comparison to a active or recently retired fighter, a situation where it is easy without closer inspection to the guys they actually fought to leap to the wrong conclusion and a situation where if you didn't see both first hand you could get it wrong.
Whcih is why it is good that most people seem to be on the right side on this one.....
Hagler was a world star, a star in a division and era of stars and Hopkins was a very good fighter in a generaly poor era who lost big fights to quite a few guys..
In the UK outside of the boxing scene Hopkins is hardly known, certainly before the Calzaghe fight, he is a star/legend in the US, but not a worldwide star like Hagler was.