According to M’nos HaLevi, the verse calls Esther a queen to emphasize Haman’s jealousy. After all, Haman was upset that his daughter was not chosen to be the queen, effectively robbing Haman of more influence on Achashverosh.
Perhaps the verse also calls Esther a queen because, according to the Talmud (Kesubos 65a), women do not generally drink – especially together with men. However, Esther’s behavior can be excused as exceptional because her status in royalty makes her an exception to the rule.
Perhaps the verse is calling Esther a queen because she was engaged in the holy work of fulfilling a prophecy. The Midrash (Tanchuma 14) applies a verse (Bireishis 49:27) that “Benyamin is a wolf that captures; in the morning it will eat its prey and in the evening it will divide its spoils” to Esther’s actions. Esther “captured” Achashverosh and Haman by luring them to a party, and then pounced. She “ate her prey” by having Haman executed (Esther 7:10), and then “divided her spoils” by carving up Haman’s property (Esther 8:1).