I don't think you did anything wrong at all. I agree with Raintree that maybe acknowledging how hard it is to lose a pet could be helpful.
Also, in general, when I want to beandip without coming across as dismissive, I try to change the subject without changing it too much. Like, in your case I might have said "so are you guys planning any home improvement projects for when the weather is nicer?" or "Did you ever decide what color to decorate that half bath?" It's steering away from the uncomfortable topic, but still kind of sounds like something you might have been planning to ask anyway.
That's more in general, though. If she was visibly upset then I'm sure she was grateful for a completely new topic.