In general, You don't need to pay/patch 'mysql' server that comes with the SnapCenter server. In future when you upgrade SnapCenter, mysql is upgraded automatically to whatever that version it supports.
However, certain MySQL versions are susceptible to multiple vulnerabilities that could lead to unauthorized takeover of MySQL Server. Therefore, it is advisable to refer to “Oracle Critical Patch Update Advisory -for specific version details that could be affected. NetApp continues to update this advisory as additional information becomes available.