Just a couple of thoughts.
With apologies to Tolstoy: All good art is alike; each bad work of art is bad in its own way.
Good art says something true about the world in a way that causes that truth to become real to us by the work’s beauty or skillfulness. It doesn’t have to be big truth; “afternoon sunlight strikes a bowl of fruit in such a way” is as true, logically, as “Christ’s resurrection is the ground of the Christian’s hope,” or “two and two are four.” “Flowers are beautiful” and “death comes for every man” are equally true. “All is not as it seems” and “these colors look striking together”? Also both true (or at least potentially, in that last case).
In that way, all good art is good inasmuch as its beauty or skillfulness or execution says something true (which is not the same as “real;” not even a little).
Bad art, though, and particularly bad “Christian” art, can fail in a hundred points. It can be vapid. It can be unskillful. It can be sentimental. It can be a bludgeon. It can be propaganda. It can say something false about the world — that the world is all sweetness and light, or that evil is an illusion. Or something false about the Gospel or about God — that the good news of Jesus is mostly about us or our comfort or eventual escape, or that God is something that He’s not. It can be moralistic. It can be pandering.
That’s not OK, even if the focus of the work is something Gospel-related.