Is balls a common noun or a proper noun?

Yes, the noun 'ball' is a common noun, a Word for any ball of any kind.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Lucille Ball
• Ball Ground, GA 30107
• Ball Corporation, best known for Ball canning jars
• "Ball of Fire" 1941 movie with Gary Cooper and Barbara Stanwycl