Zero score in Grahmaitri is not good. Grah maitri means friendship between the Rashi Lord. As you say your Rashi lord is Sun and her Rashi lord is Venus, the grahmaitri is Zero. Astrologically what it means when it relate it to horoscope matching. Sun and Venus belong to different groups. Sun is all about authority and Sun is known to be the king of planets. He is always governed by principles , where as Venus is all about materialistic desires and comforts. Sun and Venus grahmaitri is zero because they can never think alike. There are strong differences when it comes to doing things. They can never form a good team as a couple. Where as Sun will take his action based upon its principles, the Venus native will always put his comfort before considering anything else.
I hope it help you understand the relevance of grahmaitri in horoscope matching.