Also, while it's true Mize was named to the All-Star team, it was not necessatily because he was having a transcendent season.
First of all, Mize wasn't even one of the 59 qualified pitchers in baseball by July 15. He'd pitched 88 innings by then; a qualified pitcher would have pitched 96 innings.
But even if we widen the qualifying lens out to 81 innings, which results in 96 [ninety-six] pitchers, Casey would have ranked 50th in WAR, 31st in ERA, 32nd in ERA-, 44th in FIP, 45th in FIP-, 17th in xERA (pretty good), 47th in xFIP, and 43rd in xFIP-. Aside from the outlier xERA number, this is a solidly 50th to 60th percentile pitcher among all such qualifiers, not a top-of-the-league All-Star.
There are two reasons Casey was named to the All-Star team: (1) he had a 9-3 record, one of only 18 guys with nine-plus wins, thirteen of whom were named All-Stars; and (2) his team had the best winning percentage in baseball, good enough to yield six All-Stars, including Zach McStinky, of all people.