Not really, The fight turned into a slugfest because both went in there looking for the ko. Monzon was to smart and safety first for that; he'd be turning haggler all night and boxing him in the center of the ring.
Hagler did beat much tougher opposition than monzon did, Valdez his best opponent was practically a less hard hitting copy of mugabi
Quality of opposition is subjective to some extent as we cant really compare different timelines. Yes, mugabi was hard punching, but he lacked fundamentals such as a good defence. Also, Monzon handled Valdez pretty easily while haggler went life and death with mugabi in a fight that probably shortened his boxing career!
Hearns always wanted to KO his opponents, but that fight does not turn in to the brawl it was without Hagler coming out like he did.