Ichabod's mother is an "innocent child of Nature". What does this allude to?
Also, is it implied that she was insane and this was misinterpreted by his father that she was a witch?
Like the witch in the woods (Lady Van Tassel's sister) she seemed to be a "good witch", which may have appeared to Ichabod's reverend father as a pagan Devil-worshipper.
I saw no evidence of mental illness in the movie, however, simply an ideological clash (why they got married in the first place eludes me...)