I didn't find Roy's performance against Bernard all that dominating, despite what the cards suggest, nor do I think he had no problems, specifically against Hopkins. And I'm not sure where you're getting that Hopkins and Toney had such a vastly superior ring IQ. Again, just sounds to me as if you're speaking simply of the generation of fighters you know about, thus, dismissing one you don't.
For one, Roy didn't move to light heavyweight BECAUSE he was so dominant at middleweight and super middleweight. He moved up because he had issues cutting the weight, suggesting that against a Prime Middleweight Monzon, he might be apt to fade down the stretch from the fatigue of making the weight. His bout against Bernard was his absolute last at that weight, and frankly, he looked like shit during the middle rounds of that bout. There's also been noted mention of Fighters Roy specifically skipped over during those times, so perhaps his match-makers didn't feel he would be all that dominant against specific types of opposition.
It's reasonable to assume that Roy's speed would give Monzon trouble, it would give anyone trouble. I'd even be inclined to pick Roy in a decision win based on his speed and awkward attacks. But I just don't get feeling this is an easy victory. Monzon was never an easy victory for any Fighter, not one. And I don't see a reason to completely discredit one of the greatest Middlweights of all time as simply a walk-over just because we're talking about Roy Jones. Stylistically, Monzon poses a few problems of his own for Roy, he had an incredibly educated jab that he threw more than either Toney or Hopkins. He was a better combination puncher than those two, and he applied more pressure and rough tactics. His mentality was different, unlike Toney and Bernard, Monzon wasn't going to wait for you to come to him and try to out-think you. He applied intelligent pressure, baiting opposition into responses, and counter-punched well. Add in the fact that he could go to the body without being deterred much by the fast fighters he did face and their counter-punching speed, and you have some trouble for Roy.