Is it a bad sign if my baby is overweight?

Yes. Some babies may have growth problems to due with failure to gain weight or errors in their metabolism. These babies should be closely monitored by their pcp and their nutirtional requirements carefully discussed with all healthcare providers involved in their care.
Not Necessarily. It depends on the age of the baby and the amount overweight. Babies are less active early on and eat more than they utilize but as time goes on and they become more active, most times babies will got back to a reasonable weight. Anecdotally, this is especially true of breast fed babies.
Yes. Studies have shown that excessive weight gain in infancy/early childhood can persist through adulthood. Early intervention with introduction of a healthy lifestyle for your baby and the rest of the family is the easiest way to curb the huge problem of overweight/obesity. Discourage consumption of sweets/starches, monitor portions offered and mealtimes. Encourage an active lifestyle.