I see it as a crooked/crude smiley face. With the left eye overlapping the smile curve. I see it as something drawn by a kid, most likely JBR herself since it's facing the right way for her to see it correctly.
The bottom of the curve doesn't have a distinctive point, and for me, it's lacking the two humps which should be on the top. That said, if I squint my eyes I can kind of see a misshapened heart, with the center off to the right and possibly most of the ink on the top having faded or been rubbed off.
I agree that it seems more likely to be an old drawing, not something which was done just prior to or after her death.
I don't really see it as being hugely relevant to the murder, although some would like it to indicate the creepy sinister intentions of someone outside of the immediate family. Probably because Patsy and John distanced themselves from it. As per their usual MO.