Miami Marlins catcher Joe Mack (80) throws to first base. Sam Navarro-Imagn Images When assessing fantasy baseball rosters, the catcher position is often treated like a “set it and forget it” chore.  Many managers settle for a mediocre veteran who provides a stable but uninspiring .240 average and a handful of home runs, fearing the volatility of the waiver wire. However, if you want to win your league, you cannot afford a black hole in your lineup. Being aggressive with upgrades behind the plate is the “marginal gain” that separates champions from the middle of the pack.  The Marlins are doing just that, as they’ve made the decision to upgrade their own backstop situation and demote Agustín Ramírez to the minors. The 24-year-old’s .230 average and lack of power just weren’t enough to offset the defensive liabilities.…