I think the reason why most people really like a mentally messed up MC is because there's almost always a prelude required for their current predicament. The authors bother to give MC a proper backstory. In some cases it falls short because the author shows too much of the MC's backstory and in the end people are conflicted between the MC they are roleplaying as and the MC the author revealed to us. I'm reading Shepherds of Haven now and I feel this all the time. I'm not even sure what kind of person my MC is, the dialogue options and the way the conversations and certain elements of the story are written makes it difficult to find an equillibrium. With every different conversation, I feel like my MC is a different person each time and oscillating between them. Some IF's though did a good job of pulling us into that prelude without making the MC a completely different person. ITFO, Golden Rose (Imo the most immersive IF ive read so far along with Shattered Eagle: Fall of an Empire), Corruptant, BOC although we're barely out of our teens, Keeper Series did a really good job at showing the graduality of MC losing their sanity. All theses IF's have different amount of content for said prelude but they manage to stay consistent throughout the MC's journey.