It's an underperformance. The art itself has been less than satisfactory for many, myself very much included. However, it's not a flop.
And don't with these "for Gaga's standards" posts - either it is or it isn't. You don't get to redefine the word "flop" to be relative to one pop star, that's literally what the word "underperformance" is for in this field.
There was so much hype surrounding it, and her fans expect a lot from her, and she obviously had a lot of pressure to live up to. Her first three releases were HUGE and even though her main priority is the music, not the success, it's hard for part of you not to want your music to perform well. I think she tried experimenting a little bit and expanding on musical ideas she had incorporated into previous works more on this album and I don't think everyone really wanted that. I think we expected more from her. It's definitely not as good as her first three efforts. But it's not bad. I can't say it's a flop, though, when "Applause" was still a pretty big hit (bigger than some of her biggest hits), and only one other single was released from the album. For Gaga's standards, I think it was, but overall, no, there are albums that have flopped much worse than ARTPOP.
