QIf this follower is on my field, and I play Jeanne d'Arc's Fanfare ability, does this follower's "While this card is on your field, your followers deal damage equal to their defense" ability cause Jeanne d'Arc's ability to deal damage equal to her defense to all enemy followers?

ANo. This follower's "While this card is on your field, your followers deal damage equal to their defense" ability only applies to damage dealt when attacking.