Well from what their warrenty info says I would say maybe no, it does not mention it specifically. Here are a few quotes from the "warrenty void if..." section below. But regardless I don't think they can tell that you overcloked the card unless you are flashing custom bios's or something.
"Fails to return the product in the stock factory configuration or remove any aftermarket modifications."
"Causes defects through improper usage, failure to comply with operating instructions, inappropriate operating conditions, or unapproved repairs or modifications."
https://www.zotac.com/us/page/product-warranty-policy