I’d expect mermaid furniture would all be very organic, perhaps grown? I think a lot of the design concepts we see in traditional human furniture (like legs, drawers, symmetry, basic geometry) wouldn’t be present in their creations. No parallels to anything human. They’ve never seen a human home! Since transportation of large items is difficult in the water, I would think that each mermaid home would actually have it’s own furnishings growing inside it, as a natural part of the dwelling. Perhaps even just existing vegetation or features that they’ve put to use for storage and sitting? It would be both functional and decorative.

For larger items, materials would probably be a cross between items already available in the aquatic environment and the whatever human remnants float down there, I guess. I bend towards the side of fantasy, but also fantasy grounded a bit more in reality. The ocean is not a bright and shiny place full of large shells and pearls, it is deep, dark place.