Are you asking that wood make better bows than steel does, or that artifacts always wtfpwn non-artifacts, regardless of quality?
For my part, I think artifact weapons should have their "artifact" status prior to their "weapon" status, so an artifact gold sword with spikes of silk and bands of glass and a picture of clouds in bauxite would be mostly a decorative thing. Sure, you could go ahead and stab a dude with it, but why should it be more deadly than a really well made sword that's forged of steel, iron or bronze? I'd like to see artifacts be buildable no matter what, so I could erect a scepter in my mayor's office or build that one spectacular brass boot into a structure in my barracks. Having artifacts serve as "epic gear" is really secondary, a bonus. If I want spears that stab super well, I'll have my legendary smith make them out of the best available materials.
Besides, moods and possessions don't necessarily have a specific motive. They're art. Urist has a weird dream about triangles and it reminds him of the story of that time an adventurer fought a scorpion and that stimulates him to make a mica blowpipe commemorating the whole thing. He's not looking to buff up the fort's military with that, and selectively forbidding materials to guide his mood to higher dollar value and combat multipliers is a little too meta, in my view. Just let the dwarf make his lead cabinet, and then maybe put him to work in that field to get better stuff. The Gods don't always have the same goals as the player.