Your response is a list itself. See the updated code:
$.ajax({
type: "GET",
url: "/Customers/GetCustomer",
data: { Id: 1 },
dataType: 'json',
success: function (response) {
for (var i = 0; i < response.length; i++) {
$("#Customer-tabs").append('<ul><li id="li_'" i="" mode="hold" /></ul> }
}
});
-KR