I don't understand the joke since we believe Kat sings live.
Maybe Miley has insider information that Katy's live vocals are pre-recorded live vocals and that's why she said that.
Maybe she meant how the backing vocals overpower Katy's vocals? When her mic went out for that second her back up singers overshadowed her vocals so you couldn't really hear her and in a lot of Katy's performances (billboard birthday performance as an example) her backing tracks or backup singers (whatever she uses) overpowered her actual vocals
Miley girl let katy run 2014 with the #1 song of the year while you just miss opportunities to release smashers like 4x4 get it right and drive.
Ima need for you to focus on your career. Not Katy's