Something that's becoming is charming, or looks good. It's a compliment — or a sales tactic — if a salesperson gushes, "That dress is so becoming on you!"
Clothing that's becoming is well-suited to the person wearing it, flattering him and making him look great. You can also use the adjective to mean "appropriate," as when a stern kindergarten teacher scolds, "That behavior is not becoming, boys." In the 13th century, something becoming was "fitting," and by the 1560s it also meant "looking good." The opposite of becoming is unbecoming.