I can only imagine the meeting where the existence of Hunchback was revealed to Disney execs. After the failure of Pocahontas, I can imagine Disney higher-ups were positively thrilled to hear the next feature film was based on a sprawling foreign novel about rape and appreciation of gothic architecture.
Needless to say, the changes needed for the format produce an entirely different piece of work to its source; but even then it's pretty dark fare for Disney. Plus the soundtrack is probably the best non-Ashman/Menken Disney soundtrack around (Frozen is a close 2nd). If it weren't for George Costanza and his crew of annoying sidekicks, this would be my favourite Disney movie of all time.
NB: The Hunchback stage show supposedly removes a lot of the goofiness
Ashman was dead, but Menken did the music for this movie.