Is it okay for my baby to stay on their back all the time?

Yes, it's okay. Spending plenty of time on their back becomes the foundation for the next stages of development (tummy time and rolling over). In this position, visual tracking (the ability to follow moving things with the eyes) develops, both hands meet in front of the body, and your baby becomes aware of their own body. As they grow, your baby will start rolling over on their own, and tummy time will naturally increase. There's no need to force a change of position.