I finished Go Tell It On The Mountain, which is very maturely written, and highly serious. The high seriousness is something I’ve also mentioned in the past. I think it was T.S. Eliot who described it as a quality, hold on lemme see if I can find it…nope it was Matthew Arnold. He said Chaucer did not have it but Milton and Shakespeare (especially in the tragedies) did.
The high seriousness makes a work more mature. Religious questions of love and evil are serious, which is why Baldwin’s first novel was such a hit. The high seriousness, I think is synonymous with power. It’s hard to find a “funny” work powerful.
This is important to remember as a writer. Your novel should be as appropriate for an 80 year old as it is for a 21 year old, no matter the subject matter. And you can have funny, or better yet, ironic moments in a serious work. But you don’t see Hamlet laughing.