卖个萌嘟嘟嘴
[mài gè méng dū dū zuĭ]
'Sell a pout'. In this context 'pouting' (or pushing one's lips forward) is seen as acting in a cute, childlike way. It often signifies lightheartedness and fun. So this internet alias suggests a desire to display innocence or to endear oneself to others through a charming act.