No, the players have done that, in spite of a crap manager.
I like the directness of your answer. And it is not entirely without merit
Premier League finishes of 10th, 12th and 15th don't spell excellence especially when you consider the performance of other middle ranked teams like Bournemouth, Brighton, Brentford, Sunderland and at a stretch Fulham within the same period.
But to bring a bit of nuance, perhaps for the benefit of any very scared Forest supporters who might stumble across this thread - a couple of points.
Even with a good set of players a poor manager can get terrible results. I'll pick Amorin as an example. When he left and a sensible manager came in and played a sensible formation to the team's strengths Carrick got them to third. Glasner made the most of the excellent players he inherited.
Where he was incredibly lucky was that he had the players who could play the system he wanted. Or he recognised that 3-4-3 was the perfect system for the players he had. (or 3-4-2-1 if you prefer)
This was not the case for Amorin at Man Utd or it seems so far Glasner at Forest.
Finally to the point. Glasner is a good manager in good circumstances. But he lacks any flexibility in his tactical approach and mindset. Totally stubborn. And his tactical approach is extremely specific. And hard to coach.
He would only play the same limited set of Palace players because he had no faith in the other squad members being able to meet the standards required by the system. And when he was forced to play the wider squad due to injuries of the best team the results were terrible. He couldn't coach them to improve. He just got angry and lashed out.
When the better players returned the results improved and a trophy followed. So that gives strength to your statement.
But perhaps the most important argument in his favour is in the really big games, the cup games, the semi finals and finals in major competitions - he got the team (of excellent players) over the line and brought home the silverware. And no other Palace manager has ever done that.