Yeah, the plot was great BECAUSE HE DIDNT WRITE IT!!!!!!!!!!!!!!!!!!!!!!!!!!!
It's not as simple as that. Da Ponte wrote the libretto, of course, and the story had circulated way before their time, but Mozart was at that time in his career extremely picky about texts, so he suggested, criticised, changed, approved, scrapped every word Da Ponte gave to him. So credit to Da Ponte for not only being a great librettist, but also for having the patience and humbleness necessary to work with
the demon!