Helldritch
Hero
Part of the reason ol' Mordy was lame in Curse of Strahd is because you could literally swap him out for anything else and the adventure wouldn't change. It might as well have been Jeff Goldblum. There's no reason it should or has to be him. It's like a Family Guy manatee joke. "Hey! Look at this wacky out of context reference! Remember this character?"
If we're going to be more thoughtful about Acererak in Chult, it might be okay!
Yep, you did change the archmage into a generic one. You said so and you did well to do that. This is what I like about D&D. You can do whatever you want with the setting, the adventures and even the rules if you feel that you need to.
Will they be more thoughtful for the inclusion of Acererak? I sure hope so. But if they don't; I bet you that I can modify it to be great, interesting and fun (at least for my players anyways.). I have no doubt that you'll do it too.