No, materials with a cotton content higher than 60%can be called pure cotton.
In fact, there are no socks with 100% cottoncontent. Otherwise, they will not be elastic. Generally speaking, 65% of cottoncontent is good while 78% of that is high.
It's not that the higher the cotton content, thebetter the socks. Many socks of famous brands are made of just more than 50%cotton content with other contents, however, they can be more expensive,functional and durable.