No, see post above yours. I'm not even making an argument any way - I'm questioning the idea that Karl had a huge negative impact on the team resulting in it grossly underachieving.
Yes, many might agree on it, but I have yet to be told - based on what? Failed yes, as everyone on an underachieving team has in a sense failed, but "in an epic way"? Just because many agree doesn't make it automatically correct, unless there's some underlying thing you can point to.
Here are some objective, factual points: Cousins had a good year. Omri had a great year. Rondo had a great year. Belinelli sucked and we traded him (moving away from objective - but that suggests that it wasn't only a matter of being misused by Karl or that we thought he'd be much better under Joerger). Cousins and Karl clashed. We won highest number of games in years. We had a streak where we matched Malone's glorified streak.
Obviously I've been selective in choosing these points, so feel free to add on to it to support an argument that Karl failed in an epic way. Here are criticisms made of Karl last season: Karl played Seth Curry too little, didn't give WCS enough minutes and freedom to showcase his offensive ability, gave Belinelli too much freedom, had Cousins on the perimeter too much, played Caron too little and chose to go small instead. They are all valid points, but excuse me if I am not convinced that the minutes of two 3rd string (at best) players and a rookie would be the difference between us winning 30 games and making the playoffs. If I were to make an argument that Karl "epicly" failed, it would be less about the Xs and Os and more about him losing the locker room resulting in subpar effort from the players - in which case there would also be a large indictment on said players and the FO for not doing anything about it (or perhaps even aggravating it by firing an assistant coach?).
As some posters more in line with my thinking have already said - there's no way of saying that with a different coach we would have certainly been so much better. This was not a case of Karl taking over a playoff team with promise and then only winning 30 games the next season. It's not like Gentry with the Pelicans or Hoiberg with the Bulls. If that were so you'd have a pretty easy metric to point to to justify Karl "failing in an epic way".