In retrospect, that one scene is very telling of Jazz's psyche, more than any others when he's been more literal about his own feelings.
There is a scene from Thirteen when the two protagonist get so high that they start to hit each other until one of them knocks out the other one just when the mom calls them for lunch. When the girl realises the other one is passed out and full of bruises, they wear wigs, make up, and other stuff to cover it up (the mom of course notice her child has some bruises nevertheless).
Both kids dress like whores, but they wouldn't go out like
this to parties or with their friends because it's obvious they are just clowning around and they're hiding something (second img. is how they actually dress for going out). It's not any character they play, it's just for that moment. They just want to deflect how fucked up the face of one of them is at the moment. If you watch the scene, both adults present are not laughing, they're horrified and concerned.
We agree that it's odd that Jazz, being a "young woman" doesn't get a proper haircut and he dresses horrible because of his weight. He's already a "girl" but he can't perform yet as a girl. Even when he wears "normal" wigs, he's matronly and doesn't look girly girl.
And yet, she needs to be Sparkles, a drag persona. What is a drag? A man performing as a woman. He is a man who's been "transformed" into a woman and who identifies as a woman, but he hides himself from problems (in this case, the nutritionist) by taking the personality of a man who performs as a woman. In his mind, the only way for him to be a woman is to become the extreme caricature of a woman.
So, he's a man who became a woman but who acts like a man acting like a woman in order to be a woman.
The only moment when his obese body can perform properly as a woman is when he's in drag, a type of performance when womanhood can be grotesque and accepted. Just look the pic collage above: he's very "meh" about wearing normal looking wigs, but he's more excited wearing an over the top wig.
And the funny thing is that he does this when he's about to be visited by the nutritionist or confronted about his weight in the season when his mental breakdown has been more obvious. Under extreme stress, Jazz returns to his real self: a man who likes to dress like a woman, not a woman.