"Weel done, Cutty-sark!" -Mithril, Moria and Valinorean ship-building

[Morwen]

But they took Vingilot, and hallowed it, and bore it away through Valinor to the uttermost rim of the world; and thereit passed through the Door of Night and was lifted up even into the oceans of heaven.

Now fair and marvellous was that vessel made, and it was filled with a wavering flame, pure and bright; and Earendil the Mariner sat at the helm, glistening with dust of elven-gems, and the Silmaril was bound upon his brow.

Silmarillion, Of the Voyage of Earendil

It's not said here that mithril was used to make the vessel "fair and marvellous".

Originally posted by hewhoarisesinmight

Was it artistic license on Bilbo's part?

I think that this may be the case. The verses in question are made up by Bilbo, rather than a translated story, so it's quite possible that he took some liberties in his description. Take the 2nd verse of Bilbo's poem for example,... is Earendil's helmet really made of adamant? Also Bilbo's statement that "a ship then new they built for him" is at odds with the description in the Silmarillion that what the Valar did do was to hallow Vingilot itself and set it in the heavens.