What do you call the subject matter or what the viewer recognizes as the main intent or meaning of an artwork?

No, that is usually referred to as subject matter. A meaning or message in a work of art is more subjective, that is it can be different to different people. A simple example is the statue of 'David' by Michaelangelo. The subject matter is a young David at the moment before he slays Goliath. A meaning or message of the work could be the super human qualities of a hero.