Moment can also mean "importance." If you say something is "of little/small moment," then you are saying it doesn't matter. (Vocabulary.com)
In indeed, Merriam-Webster also notes this meaning:
importance in influence or effect
- decisions of moment must be made by our government (L. H. Evans)
Therefore, the sentence "The question is one of small moment" means that the question has/is of little (negligible) importance.
As for the other statement, "Certainly good art pays as well as mediocre" states that good art is not better paid than mediocre art. You might have been confused by "the art pays well which is an instance of middle construction, described by some as "active in form but passive in meaning". E.g.
- Sports cars sell quickly.(meaning: Sports cars are sold quickly)
- The woolens wash well. (meaning: The woolens are washed well.)
Therefore your sentence has the meaning:
"Certainly good art is paid as well as mediocre art. (the middle construction "good art pays well" is more idiomatic, as Gngram confirms).