That's not necessarily true. "Bless your heart" can be endearing as well, but it depends on when it is said in the conversation referenced. Normally it depends on whether you say it at the beginning of a statement or after. For example:
Bless her heart he husband left her. That's empathetic.
He can't find his behind with both hands, bless his heart. This implies stupidity.
“He can’t find his behind with both hands, bless his heart. This implies stupidity.”
It can also imply affection despite stupidity.