In the same way curling in a ball conserves heat, sleeping with an exposed belly helps a dog cool off. Since the fur is thinner around the belly and the paws hold the sweat glands, exposing these areas is a great way to beat the heat.
It’s also a position that indicates a dog is very comfortable, leaving their most sensitive areas vulnerable and making it hard to get on their feet quickly.
A pup who’s sleeping in this position most likely doesn’t have a care in the world. It’s common in summer months.