Is the word interesting a common noun?

No, the Word "interesting" is not a noun at all: it's an adjective, a Word used to describe a noun. Adjectives can describe what kind, which one, or how many."interesting" is a what kind adjective a noun is a person, place, thing, or idea.

The Word "interesting" is the adjective form of the noun interest, a common noun; a general Word for curiOSity or concern about or attention to something; a general Word for a charge for borrowed money; a general Word for a right, claim, or legal share of something; a Word for a thing.