The term "mbn" seems to be a typo or a slang abbreviation for the phrase "must be nice." This expression is often used to convey a sense of envy or sarcasm towards someone who has something desirable or fortunate. It implies that the situation being described sounds pleasant or favorable.
The abbreviation "mbn" originates from internet slang and texting culture, where brevity is preferred. It is a shorthand way of expressing a feeling that is easier to type compared to the full phrase "must be nice."