Does mouth-breathing cause a sore throat which came with a fever?

Dryness: Mouth-breathing most commonly results in dryness in the mouth and relative dehydration. It should not, however, give anyone a fever.

Yes and no: Mouth breathing commonly causes a mild sore and dry throat that will improve with hydration. It by itself doesn't cause fever. A viral nasal infection will congest the nose making it easier to mouth breathe and cause fever but these symptoms are temporary once the viral illness runs its course.