I think she said it because she understands that Yuuko could actually defeat her if she was a bad girl. And is also probably a way of being nice, as Yuuko always says this.

I liked this episode a lot. But i felt it was extremely rushed. So much dialogue running so fast, i could barely read the subs. But even so, i thought it was the most precious episode so far.

Now, scanlators, please translate the other volumes of the Machikado Mazoku manga. I never asked you anything.