I've often used the term clean line to mean a certain design aesthetic ... I don't think of myself as pretentious!
Clean line isn't the opposite of dirty line, or hairy line- just that the line isn't busy, and it's well, clean and sweeping, airy and uncluttered.
For any sport that requires an extension of the line it makes sense not to have a floppy uniform, and to have attributes to the costume that draw the eye along the line... Think the placement of sparkles, stripes or other elongated prints.
A big old mess would be blocks of colour, camouflage and loose fabric. These keep the eye in a small space, and there's no payoff for extending. The enjoyment you get for watching and participating in Ballet, gymnastics diving and swimming is all about that extended clean line, and the flexibility and strength which enables the flow of the line. It's why we love dolphins.
It's pleasurable to look along a line and for it not to be cluttered or busy. That's why people play golf I think- it's good at relieving anxiety as the fairways have clean lines. It's why people pay declutteres to sort out their flat surfaces!
There are plenty of other examples of the aesthetic qualities of clean lines.
The development of Perspective was absolutely mind blowing. It's why the cubists were reviled and modern art (seeming to have no structure or line) was so radical and still is an acquired taste
I'm delighted the American women's gym team won so many medals, considering the aesthetic of gymnastics was predominantly white and wasp shaped. Having muscular black, Jewish and Hispanic women on the us team is more of a game changer than their uniforms.
Black women are also making headway in ballet also, and this is something to be embraced as girls with bigger muscles and curves were traditionally excluded.
Sorry if that all sounds pretentious ... I work in design and clean lines (and negative space) matter to me.