It wasn't the Dark Phoenix saga for sure but it definitely wasn't the Phoenix saga either. In the early days Jean had problems with the rushes but she wasn't perceived as a villain or a threat to the world. Jean as Phoenix was a hero and though Scott and some of the others were worried about her, she did good things.
The movie tried too hard to be something it wasn't. They should have learned something from the Marvel movies and it should have been unapologetic for being a super hero movie. It needed big effects and it needed to be true to the source material. It could have been something great. Instead we had a watered down version that felt deeply uneven.
Again I like Sophie and I did enjoy the movie. I felt for Jean in several instances but it is obvious the mass audience did not.