From the start of the season through February 26th of this past season, Boston was the top point generating team in ATO plays at 1.28 points per possession tied with Dallas. I haven't seen data compiled for the full season, but I suspect it isn't much different than it was at the end of February.
In other words, Joe's ATO play calling was good enough to lead the league (even accounting for ORTG relativity, Boston was 5th).
This thread is full of perception that isn't borne in reality. Actually pretty typical of internet blogs and forums in general.
https://www.basketballinsiders.com/news/nba-study-which-nba-coach-is-the-best-at-calling-plays/?fbclid=IwZXh0bgNhZW0CMTEAAR3uTDcyE2VGb9fxAOfejFsHXaNnRK5M1gwG4mlv9n6AOSdzwqr6wJRTu90_aem_jIk-9Vq6ERyyH-VM4sjlHw
That's interesting, I wouldn't have expected that. TP.
Do you have similar stats for end-of-quarter plays?
It's because human beings tend to remember things that fail, or that didn't get done, rather than things that actually did. If something worked you move on to the next thing. But if someone did do something they should have done, you definitely remember it 
That's probably why Brad wasn't worried about Joe to the extent the fans were...he looked at the data, rather than whatever anecdotal evidence he saw in games.
You also need to remember the nature of ATO plays. They're not the same thing as game-winners or end of quarter shot. An offensive possession after a timeout in the middle of the quarter counts, too. In fact, those are probably the vast majority of ATO plays.
Therefore, it makes perfect sense that the team with the best offense in the NBA would have the best offense after a timeout is called. That's something the authors of the article attempt to adjust for, and it leads to the Celts falling to fifth (tied).
But, ATOs are different than specific plays that are set up by the coach in crunch time, which is what the complaints seem to be about. I think the flip side is the pre-planned "leave the ball in the star's hands" at the end of quarters / games strategy, even when a timeout isn't called (and therefore isn't encompassed under ATO plays).
People want diversity in those situations. We've all seen the "give JT the ball to run the clock down and chuck up a buzzer 3PT miss" play, and it is definitely the team's go-to.