I don't recall those words exactly, but I do recall reading that Patsy used to draw a heart in JB's hand. I think that was mentioned in ST's book.
What's odd is that, if she drew this particular heart, why would she say that it was "a pretty good little heart"? She wouldn't say that if she herself drew it, and it doesn't look like a heart drawn by an adult. To me, it's almost as though she was admiring the art work of one of her children. Which is exactly what it was, imo.
Actually, I think she'd say it to try to give the impression it was "too good a heart" to have been drawn by a child. She was trying to give the impression that whoever killed her (the intruder/sff/disgruntled employee/housekeeper) drew it.