Is that a good idea to make a response regarding user actions like this one? When user about to unsubscribe changing the face to sad and when click cancel makes face happier. Is it good practice?
Not necessarily a "good practice" as the only result would be to reduce drop-out-rate, so it does not provide any benefits to the user but it would also not bother him if this doesn`t tip over to become a dark pattern.
An added value to avoid to use them is to think about cultural differences among global online users and keep in mind the international usability testing meaning. Not everyone reads the emoticons in the same way, so I would avoid to use them otherwise if really want to use let them match with the user's cultural background.