A newborn baby can sleep in just a diaper as long as it's warm enough in the room.
But during the winter months there can be drafts in the room that you may not feel but the baby may feel it and feel colder.
So during the winter months I would always keep a pair of footed pajamas on my babies when they were a year old or under.
During the spring and summer months I would let my newborns or toddlers sleep in just a diaper which they felt more comfortable in.
Also when my kids were 2 years old or older they would sleep in just diapers and maybe a shirt during the summertime when it was hotter in their room.
They would even go around the house and yard wearing nothing but a diaper during the day as they were more comfortable that way as well.