People forget that she's the same person who made The Fame Monster and Born This Way. She hasn't suddenly stopped being good at making catchy songs. She'll stumble and do better some times than others just like other songwriters do. She doesn't have the luxury of being a consistent machine like people like Rihanna, who can just hire the hit DJ/writer/producer of the season and get a sure-thing bop out of them.
At the very least, her saying that ARTPOP was a good album shows me that she at least gives her all when she releases her albums, and genuinely believed in it. If she was like "yeah it sucked, sorry" then the whole album campaign in the first place would come off as ungenuine... like she was trying to sell something she didn't give two shits about and failed.
All of this will be irrelevant anyway when she comes back and clocks the top of the charts again, because you KNOW she's more than capable of doing that. Denial is just more fun