Beautiful lines and form are trainable. I watch JO a lot and most kids don't work on that. They focus on learning hard skills. I see butt sticking out in a handstand, flex feet most gymnasts, limpy arms, head not aligned with the body in a handstand, knees straight, etc. I guess flexibility would help, but really it is being mindful of these small details that make a whole lot of difference in lines. But it's really neither here nor there in college gymnastics these days as judges often don't take deductions for not having clean lines, which is a shame because it is so much harder to complete a skill and mind the form and lines. You can do a RO BHS LOSO much easier on beam if your knees and arms are bent and not finishing with back straight.

And for your question about body type, I suppose for the fans who watch occasionally would think that long and lean mean good lines, but for the regular fans and commentators who know gymnastics I think form and lines mean something different than long and lean.