Gaga's decline had already started with Born This Way, though. The title track was a hit, but a lot of it was due to hype. The other singles failed to have the impact of the singles off her first 2 albums (she had actually built momentum with The Fame Monster). So, when ArtPop dropped and was even more of a disappointment commercially, it was like a nail in the coffin in a sense.
The rot was starting on BTW for sure. While I enjoyed it, the quality control was going in Team Gaga after the first single off that album. The music videos were getting muddy and stupidly indulgent or just poor quality. Gaga was starting to try too hard and someone should have told her 'no' here or there. Unfortunately, this didn't seem to happen and we ended up with Artpop - all the worst factors building into one horrid result.
Messy. Disconnecting. Forced. Clunky.
I hope the project taught her a lot of lessons. It was a misfire on nearly every level.