I think "The weather is terrible." should have been accepted.
It seems that there can be a variety of negative meanings for "ddiflas" (terrible, miserable, depressing, ... ?) rather than one direct translation?
ofnadwy is more usual for 'terrible'.
Many Welsh words have several English meanings, and vice versa. However, we try and pick no more than the two or three best or most common ones in the course translations - if we tried to include them all we would end up with a large and unmanageable thesaurus instead of an introductory Welsh course.