We are mixing two different sources of information here about "Brokeback". Two different canons, so to speak.
a) the short story
b) the film
A.L.'s film makes some departures from the A.P.'s short story. I think that each needs to be viewed on its own merits, not blended together as if they are one entity. Mixing them together can make for a muddled analysis.
a) In A.P.'s short story, Jack had buck teeth, was short, and had a big butt.
This is how A.P. described Jack Twist,
"Jack seemed fair enough with his curly hair and quick laugh, but for a small man he carried some weight in the haunch and his smile disclosed buckteeth."
A.P.'s Jack was concerned about his appearance for very good reasons, his description in the short story shows us that he was not attractive as a man. Of course, there are exceptions, but gay men don't tend to find a big butt on a man to be attractive. Gay "urban legend", if a man has a big butt, he is probably is not gay.
Jack had his teeth filed down to make himself more attractive, probably not for Ennis, Ennis already knew Jack had buck teeth. Jack probably did this to make hunting for "tricks" in roadside parks and other closeted sexual arenas easier. The mustache was popular on men in the 1970's, I grew one for 6 weeks and hated it, shaved it right off. Jack's mustache is consistent with the period.
A.P. describes Jacks appearance as changing over the years, "Jack had filled out through the shoulders and hams" He was getting fatter, for a short man that is not a good thing. Jack filed down his teeth and grew a mustache, "he had his front teeth filed down and capped,..., and to finish the job grew a heavy mustache." The only mention of a growth on either of the boys faces is the one on Ennis's eyelid, "A benign growth appeared on his eyelid and gave it a drooping appearance..."
The short story doesn't say much about Lureen's appearance.
b) In the film we are treated to the appearance of Jake Gyllenhaal as Jack. Quite a change from the image I had formed reading A.P.'s story!
In appearance we see him grow a mustache, gain weight, and sport a clunky Rolex wrist watch. All just about what I would have expected to see on a Jack Twist aging thru the 1970's into the 1980's.
We also get to see Lureen, and her transformation into a successful small town Texas farm equipment executive. She favors the slathered on makeup, the peroxided hair- do, the loud, large pieces of jewelry. Her nails are brightly painted. Her teeth begin to stain from the cigarettes. In short, she want to looks the part of a successful business woman from her small town in Texas. And, A.L. got that image right in the film! That is exactly how I remember the appearance of most successfully business women in Texas in that period.
Why did she peroxide the hair? "Blondes have more fun!" And as Texas women age, they just get bigger, blonder hairdoes! It is also would have been easier to cover up any gray with peroxide, rather than using the natural chestnut brown color of her hair.