Agree with Ronen.
To keep a track of supplier's ISO certificate or not, is part of the risk based approach in your Purchasing Controls and is based on the risk the purchased product has on the safety and performance of the medical device or the suitability, adequacy or effectiveness of your QMS.
Typically, the purchasing process involves selection, qualification, and monitoring, and your scenario applies to the "monitoring" phase. As per the new ISO 13485 Practical Guide, you could use incoming inspection, auditing, and feedback processes too to support the monitoring phase.