As many posters have mentioned, the phrase “Miss Thang” is meant to describe someone of feminine nature (whether male or female), who acts or believes themselves superior to others. It is usually said mockingly or as a derogatory description of that person. Although, like many derogatory words/phrases, it is often used between friends as a form of camaraderie. Unless you are very familiar with the person you are addressing as “Miss Thang”, you should be careful with its usage. It might be taken as an insult.
I think the phrase “Miss Thang” has its origins more aligned with US Black dialect or Ebonics than it does with gay culture. I’ve heard this phrase my entire life from the early 1970s. I did not hear it applied to a gay person until a gay person said it about another gay person in the 1980s. My parents and grandparents would use the term as a common phrase. So, its origins probably date back two or three decades or more before the 1970s. It was said in derision about women who were overly self-important or showy with their clothing and/or demeanor. It was also said as a form of praise to a female child who performed well at a task or made themselves exceptionally presentable.
“Look at Miss Thang over there trying to be cute in those whorish clothes.”
“Look at you, Miss Thang. You played that recital like the next Beethoven.”
One poster has posited that the phrase might come from another phrase, “is something”. As in, “She thinks she’s something.”, or “She is something. Isn’t she?”, or “You sure are something!”. Words like “important”, “special”, “else”, etc are implied after the word “something”. Phrases such as this and their derivatives can be either positive or negative depending on there connotation and context. This theory may have merit.
Although, the phrase can be attributed to either gender for adults and adolescents. It is normally reserved for only female children. Male adults and adolescents are normally called “Miss Thang” when they show a tendency to be very effeminate. Although, gay men who are not necessarily effeminate are sometimes called “Miss Thang”.