Yeah, I would call a 7.6 bWAR year, where a guy wins MVP "amazing."
Let's rank the top 14 seasons by Mattingly (M) and Hernandez (H) by bWAR.
1)H: 7.6 2)M: 7.2 3)H: 6.7 4)M: 6.5 5/6) M/H: 6.3 (tie) 7)H: 5.5 8)M: 5.1 9)H: 5 10)H: 4.6 11)H:4.4 12/13)M/H: 4.2 14) H: 4.1
That's 9 for Hernandez and 5 for Mattingly.
Now you will say, but Hernandez didn't hit for power. This is true, but he was arguably the greatest fielding first baseman ever.
Now you will say "But I'm smarter than WAR, and I, unlike WAR, know that being a good first baseman doesn't matter."
I would say that usually it is hard to have a large fielding impact as first baseman, unless you are Keith Hernandez. He was just that good.
An accepted standard for defensive value is total zone defense (Rtot). Though, imperfect, Rtot shows the importance a first baseman can have. Keith Hernandez' Total Zone Defense was 117. He is
40th all-time of any player ever going back to the 50s when the stat measures to.
By comparison, Bill Mazeroski's Total Zone Defense (rTot) was 147, Omar Vizquel's 129, Mike Schmidt's 127, Don Mattingly 33, and Will Clark's was 2.