"Became mellow" is a phrase used to describe the process of becoming more relaxed, calm, and easy-going. There are many synonyms that can be used to describe this state of being, including "chilled out," "mellowed out," "laid back," "easygoing," "calm," "serene," and "tranquil." These words all convey a sense of peace and relaxation, and can be used in a variety of contexts to describe a person, place, or situation. Whether describing a beautiful sunset, a peaceful yoga class, or a person who is always relaxed and easy to be around, there are many synonyms that can be used to express the feeling of becoming mellow.