Chan Wei Lik wrote:

Urd never could lie very well. Being a goddess had certain disadvantages.

Actually, Urd is half-demon. And she does lie well. Getting away with it is another matter.

That's the point, she can't get away with it.  All her lies are
monitered by Kami-sama.  A master of lies, a succubus, like Morrigan
could detect someone that had to be careful of her words.  Too many
lies, Urd loses her powers.  She just got her powers back in the manga,
and I don't think she wants to lose it again.