Adding new art, certainly does take developer time. Tile art properties, house add-on plugins, naming...some or all of these tasks have to be done just as "new art" housekeeping, not to mention whatever additional tasks are required to make whatever that art is actually function the way it's supposed to.Art assets don't take much in the way of "dev" time, just the time of the artist.
I'd love to know what Onifirk does at this point, as the volume of new art per patch is tiny/nonexistent except for around anniversary and Christmastime.
However, adding new wearables to the CC is probably never going to happen since they lost the original character models to build it on.
In 2014 approximately 264 pieces of tile art were added, as well as ~20+ pieces of gump art.
That all being said, new wearables is something we've been wanting to add for a while and I wouldn't be surprised if you saw some in future publishes