Its a tough crowd, or uniquely unreasonable conviction, to label Cookies lesser seasons as average to mediocre without context.
In his first season in the rotation, 2011, he pitched much of the season with a sore arm that led to TJS in September.
In 2013 in his comeback from surgery he only threw 46.2.
Over those two partial seasons his FIP was fairly average, about 4.20.
In 2014 he spent time in the pen before rejoining the rotation. From the beginning of that season until his cancer his ERA+ by year was...
153
119
136
139
128
In his cancer season it was still 91.
And in his return to the rotation in 2020 it was 159.
Those numbers are not merely average, let alone mediocre. He ranged from 20% to 60% better than the average MLB pitcher...except in his cancer year, when he was 10% below average.