A key performance indicator (KPI) for customer support feedback could be the customer satisfaction score (CSAT). This metric measures the level of satisfaction customers have with the support they received. It is usually obtained by asking customers to rate their experience on a scale of 1 to 5 or 1 to 10. A high CSAT score indicates that customers are happy with the support they received, while a low score may indicate areas for improvement in the customer support process.