Narcissa absolutely loved animals. All kinds; from giant dragons, to her evil little cat, right on down to mice. She clapped her hands together in pure delight when she saw exactly what was in the box.
"They are adorable, Reg! If you send them into the Gryffindor common room, they might get hurt. So, maybe we ought to put some kind of protection spell on them?"
However, the idea of all those 'brave' Gryffindors going crazy over a few mice, was just plain hysterical. What was so frightening about mice, anyway? They were awfully cute and really did no wrong at all.
"Could I have one to place down the back of Ugly 'Ol Umbridge's dress. Or perhaps in her bookbag if she leaves it laying around."
Narcissa found Dolores Umbridge to be quite unpleasant in all regards. Plus, Narcissa did have this terribly wicked side to her, which caused her to enjoy making fun of others, though, she rarely did it to their face. Instead, she would sit to the side and feel silently superior.