Is cotton an adjective

No, the Word cotton is a noun, a common, concrete, uncountable noun as a Word for a plant grown in warm regions that has white fibers used for making cotton cloth; an uncountable, material noun as a Word for cloth made from the white fibers of a plant; a countable noun for articles of clothing made from cotton fiber.

The Word cotton is also an informal verb meaning to begin to understand, to begin to like someone or something.

Although it is not an adjective, the noun cotton is often used to describe another noun, for example a cotton shirt or a cotton dress. This use is called an attributive noun (noun as adjective) or can be considered a compound noun, such as cotton balls or cotton candy.