He's most likely being sarcastic.
I'm having trouble finding anywhere else where the cat uses that kind of direct sarcasm, but such sarcasm definitely fits with the style of ironic verbal abuse that comprises most of his narration:
But however ugly I may be, there’s no conceivable resemblance between myself and that queer thing which my master is creating [...
The novel may be "The Inimitable Jeeves" by P.G. Wodehouse. It concerns mainly the events associated with a secondary character, Bingo Little, who falls in love with almost all the women he meets. He is the target of scorn from the principle character, Bertie. From memory, I believe he makes the reference regarding seven-tenths of women.
The passage ahead of this is implying, by showing how the cats react to the cruelty of humans, that the cats take on the traits of their human masters. Miss Blanche, who lives with the military man, suggests making war on the humans:
Miss Blanche, having given through her tears a complete account of this event, assured me that, to maintain our own ...
Because O-san was told to yank the rice-cake out of the cat's mouth, though she wanted to see the cat dancing longer.
The original Japanese sentence is 御三は御馳走を半分食べかけて夢から起された時のように、気のない顔をして. I think "listless" and "half-hearted" are appropriate for 気のない rather than "vacant".
I think the "matters" being referred to here are not just about robbery, but earlier text as well. Reread the lines directly proceeding your quote:
Miss Blanche, the white cat who lives opposite and whom I much admire, tells me whenever I see her that there is no living creature quite so heartless as a human. The other day, she gave birth to four ...