Grandal's WAR is 0.9 in around 100 ABs with a 146 wRC+.
Mesoraco's WAR is 0.1 in around 180 ABs with a 64 wRC+.
You're talking almost a win difference in almost half the ABs offensively. Grandal's offensive season is that (so far) of a difference-maker at a premium defensive position. Mesoraco is replacement level. No amount of "bad luck" Mesoraco's had touches that particular difference.
Now, maybe Grandal's playing over his head. Maybe Mesoraco has another massive improvement left in him. Maybe it is a lack of playing time that's hindering Mesoraco's development. Maybe Grandal gets injured because he's taller than the average catcher.
But those maybe's don't mask the fact that Grandal's been the better player at the major league level this season by a wide margin.