In Magento 2 Cart Product Comment allows customers to add comments for each product in the shopping cart while editing products on the cart page.
Customers can leave a comment on each product in the cart. Cart comments are visible on every order in the backend for admin users. Cart comments are visible on every order in the “My Orders” section in the frontend. Cart comments are also visible on the Order confirmation email for customers. Admin can view customers’ comments and take the necessary actions if needed. Admins are able to see each comment in the order view section.
For more Info: Magento 2 Cart Product Comment Extension