The team he pitched for is irrelevant. The CY isn't for the most valuable pitcher, it is for the best pitcher, and SPs rightly get more respect than RPs because RPs have always been failed SPs. Mariano had a great year last year, but Johan had a better year IMO because he is a starter.
Yes, Gagne and Eck won a CY, but Eck didn't deserve it in '92. He deserved it in '90, where his ERA was 0.61, his ERA+ was 606, and his WHIP was 0.61, and I think the sportwriters were just correcting a mistake. Gagne won it with a 1.20 ERA, 335 ERA+, 14.9 K/9, 0.69 WHIP, and 55/55 SV. Mo's ERA and ERA+ stats were close to Gagne's, but his peripheral stats weren't and he wasn't perfect. They have set the bar for relievers. Mo didn't clear it.