The artwork for each legendary item is fixed. Once you see what one level 95, third age, crafted, lore master staff looks like - you know what they all look like. That one looks better to me than the butterfly Lore Master staff. Turbine seems to like putting animals on top of Lore Master staves.
There is no cosmetic weapon system. You have to accept what ever appearance Turbine decides is appropriate for your legendary weapon.
Unless stated otherwise, all content in this post is My Personal Opinion.