Encontre estas explicaciones. A decir verdad soy traductor técnico, y jamás había escuchado esta expresion.
People who wear their heart on their sleeve express their emotions freely and openly, for all to see. Example: "She is kind of player who never hides how she's feeling. One look at her and you know if she's winning or losing." Reply: "She wears her heart on her sleeve."
You can see how these people feel as easily as if they were "wearing" their "heart" on the "sleeve" of their shirt. Example: "Some people love him and some people hate him, but my brother always lets you know how he feels. He wears his heart on his sleeve."
People who wear their heart on their sleeve do not hold back their emotions, for good or for bad. It is clear how they feel in each moment. Example: "She's a shy person. She's never been one to wear her heart on her sleeve."
Estos conceptos me hacen pensar en "corazón transparente" La pregunta es si se quiere decir en forma un tanto poética o más bien coloquial.
En forma coloquial diría "es un persona emocionalmente abierta, extrovertida" En el norte de México decimos que es una persona "francota, que no se reserva nada"
En forma poética diría corazón franco, abierto.
en verdad por más que lo pienso, creo que no tenemos en nuestra cultura un concepto equivalente.