Mattingly's direct involvement with the hitters is completely unknown. His title is "offensive coordinator", and they have two other clear "hitting coaches". It is obvious that he isn't working day to day with hitters in what would be assumed to be the "hitting coach" duty.
This has become a meme at this point. I'm really not sure what value he brings to the staff, but its a bit silly to pretend like any time a hitter is doing poorly it is somehow his fault, but if a hitter does well it is to someone else's credit. He either has direct control of the hitters, or he doesn't. Lets not pretend like Mattingly is directly "coaching" all of the poor hitters, but Matt Hague is "coaching" all of the guys who are hitting lol.
I've been saying Mattingly is a problem long before it was picked up with others. Since he has been here the team's power and hitting ability declined. Look at last year and many players saw a decline in production. This continued on into earlier this season. Before he got here the team was among the top with slugging percentage. There would naturally be a decline in power due to the moves they made, but it was drastic. At one point this year they were I believe 29th in slugging percentage.
His philosophy isn't current with modern baseball. Donny isn't keen on doing damage. Current baseball is all about doing damage for the most part. Analytics is a big part of the game, and he doesn't strike me as analytical.
Now, we don't know what goes on behind the scenes but we can get clues from broadcasts and articles. Hague was the first one to setup the pitching machine to mimic the starting pitchers repertoire. I am not giving Hague all of the credit. He's probably working with Davis and that's not working out. However, Springer and Loperfido are recent players who the broadcast has said have been working with Hague and they have had success. Hague has been more involved and visible on the bench and the bats have improved. A few days ago, it was mentioned that the Jays bats have a different approach. Hitters want to do damage on certain pitches. This goes against what Mattingly was preaching early on.
It would not shock me that IKF had success with Mattingly because that's the type of player Mattingly wants to work with.
Overall, since he's been with the team, offensive production has dropped a lot. In no way does he deserve to be here next year. He has had a poor track record.