I see no reason not to allow "frosts". The french is plural and the english can be too
It would seem reasonable to accept either and teach both, as Duo does with some other expressions. By the time someone gets to this stage of the course they will have noticed that Duo has this approach, and, hopefully, be able to benefit from it.
The subject is 'Morning Frosts' (plural) The verb is 'are' (plural) The answer could also be 'Morning frosts are not rare' But Duolingo is free and that's amazing!
Because in my limited understanding, "du" means "of the" so it literally translates to "frost of the morning" the focus is on the frost and not the mornings, but like I said limited understanding, just trying to explain how I understand