"Bless your heart" is sort of an all-purpose expression. It can be used to mean anything from, "you sweet thing" to "you're an idiot".
For example, "She takes such good care of her elderly momma and daddy, bless her heart." or
"He doesn't have the sense to come in out of the rain, bless his heart."
It is just one of those sayings that you have to hear in context to understand what meaning is intended.