Of all the questions that people ask, this is one of the most difficult to answer. What exactly is a "good" baby anyway? Have you ever met a "bad" one?
One person's "good" baby might be another person's headache. For me, a baby who wakes up every few hours at night to nurse is entirely normal and good. But other people expect their babies to sleep long stretches at very early ages. I am also used to babies who nap a lot during the day, especially when they're little. But my youngest sister, whose baby is just 2 days older than Inga, has a baby who literally stays awake all day long. He might nap once if she takes him for a long walk. (Then he sleeps 6-8 hours at night, wakes up to nurse once, and sleeps the rest of the night.) Good baby? Bad baby? Who knows...
The words "good" and "bad" are far too vague to describe an infant's attributes. I prefer specific ones with a generally agreed-upon meaning. Dio, for example, was definitely a fussy baby from about 2-8 weeks old. Not "bad," just cranky and difficult to console at times.
So how do I respond? I laugh and deflect the question by saying, "I don't know. Is there such a thing as a bad baby? I think all babies are good!"