Some options that won't require replacement of the drywall sheets; If the room (drywall) is not textured then the joints, seams and screw heads can be coated with drywall mud and then sanded for a better finish. Make sure to prime first before applying drywall mud to keep it from bubbling and help it stick.
Or, a texture can be added to help cover up the walls and ceiling. There are many types of texture that can be used. Check-out this article for options- <a href="https://www.house-painting-info.com/drywall-textures.html">Types of Drywall Textures</a>. The thicker the texture the more it will cover but there are limits.