Come again? Dak has his two best seasons under McCarthy, Rodgers developed a HOF resume under him. Aaron Brooks also played his best football for MM.
I already did Brooks. Here's Dak by the numbers, courtesy ESPN:
Coach GP CMP ATT CMP% YDS YPA TD INT LNG SACK RTG QBR
Garrett 16 311 459 67.8 3,667 8.0 23 4 83 25 104.9 77.6
Garrett 16 308 490 62.9 3,324 6.8 22 13 81 32 86.6 70.0
Garrett 16 356 526 67.7 3,885 7.4 22 8 90 56 96.9 55.2
Garrett 16 388 596 65.1 4,902 8.2 30 11 62 23 99.7 71.9
McCarthy 5 151 222 68.0 1,856 8.4 9 4 58 10 99.6 73.1
McCarthy 16 410 596 68.8 4,449 7.5 37 10 51 30 104.2 60.5
McCarthy 12 261 394 66.2 2,860 7.3 23 15 68 20 91.1 59.9
McCarthy 17 410 590 69.5 4,516 7.7 36 9 92 39 105.9 73.4
McCarthy 8 185 286 64.7 1,978 6.9 11 8 65 21 86.0 41.9
McCarthy 17 404 600 67.3 4,552 7.6 30 10 86 31 99.5 70.3
In these numbers there is ZERO evidence of Dak getting better after McCarthy took the reins. He was a very good QB well before McCarthy set foot in Dallas. He was also better in the playoffs pre-McCarthy. The rating numbers are unchanged, with the expected variation year to year, with QBR higher on average in the Garrett era.