Thinking more about it: at least stereotypically, cat people tend to be more individualistic, and dog people tend to be more like pack animals. Research has shown that to be true, at least to some point.
I could definitely see many of Katy's songs, especially her deeper songs, appealing to more indidualistic people, who would also be more likely to be cat people. That might explain some of the correlation, if such a correlation does exist.
One common misconception is that many people think that cat people dislike dogs and dog people dislike cats. That is not true. The truth is that it's more common for cat people to dislike dog people, and for dog people to dislike cat people. That is likely because the personality associated with cat people (individiualistic) and the personality associated with dog people (pack animals) tend to clash with each other.
I hope that Katy keeps that cat section of her tour when she is in the US and continues to dress up as a cat and stand on the yarn ball.