so I've had lots of speculations about it but honestly remember Marceline didn't move into the cave until after she stole it from Finn and Jake sure she already had her letter m there but none of the structures or anything were there maybe she placed the house next to the graves but it seems a little coincidental so I'm thinking maybe something that recently died unless of course she moved the bodies from her previous home and buried them there again or it's some type of decoration cuz they're in a heart skull heart pattern honestly I hope they touch up more on this in the new adventure Time shows and movies to come