In our age of increased acceptance of showing cleavage; and fashion that, in general, makes it difficult for women to avoid immodesty, we have grown quite accustomed to women displaying skin.
The result is a desensitization to the appropriate form of attire when working with, for example, children. And the more pornography culture influences our thinking, the more accepting we become of promiscuity.
So it probably never occurred to Ms. Perry, or the person shooting the video, that her dress was inappropriate. After all, she always dresses that way, so what could possibly be wrong with it?
The more we see it, the more normal it becomes. And those who would hold a more conservative view of how Ms. Perry should dress are worthy of ridicule on Saturday Night Live.