Is the word mainstream a noun?

Yes, the Word 'mainstream' is a noun; a singular, common, abstract noun; a Word for a prevailing current or direction of activity or influence; a Word for a thing.

The Word 'mainstream' is also a verb and an adjective.

Examples:

His designs are outside of the mainstream of fashion. (noun)

Corporate America has begun to mainstream energy efficient products. (verb)

He's a favorite candidate in mainstream politics. (adjective)