I've actually talked a bit about this before (somewhere back in the 70ish pages, normally I'd go find the post and link it, but, uh, working on the update).
The short version is, yes,
Domdeyso is correct I am adapting something I have no input on (and fucking with it in a way the original dev never intended). In the Unity version, Morticia really gets ignored pretty hard (she has a third of the number of scenes that Summer does), such that you can play a LOT of the unity version without ever seeing her.
I've done my best to increase her presence throughout this game (she was the first (and only for a while) character I made additional scenes for), but, I am limited by what I have to work with, both in terms of story and art (my ability to make art is, not so great).
So there are parts where her presence is... lacking. You're not the first to point out her mid game drop off (although, I think you'll be happy with the alternate scene in Day 28 (again, not in the unity version)). Mostly, Morty's attitude towards her is due to a lack of scenes with her. Morty more or less has to turn her down, because this game doesn't have any images of boats- er, I mean, only has limited scenes.