No not necessarily "no good" but he is old school - "short to the ball", just do a google search, there's a recent article on Fantragphs (I beleive it's from 2021 - he tells you his philosphy).
Also this from 2018
"Why Marlins Manager Don Mattingly Doesn't Buy Into "Launch Angle" | The Dan Patrick Show "
There's also a piece on Chris O'Leary's site where he breaks down Mattingly's swing (not his philosphy), showing him trying to create big lunch angle. So maybe Mattingly just can't explain what he does naturally at the plate.