So I unmolded the dwarf statue I cast a few days ago, and as I feared, it came out pretty bubbly.
I was able to fix the worst of it with spackle, but it still looks a bit worm-eaten (which is fine; as I said, it's going into a ruin anyway). Still, I'm encouraged enough to keep at it, and the the smaller test pieces I cast (including some shells and some rocks) came out perfectly.