>>10910746oh, I thought you were talking about overweight/obese apple shaped bodies using corsetry to create a waist.
For healthy weight people who don't have a waist, I believe that's called a "fridge body" or something along those lines here but I got used to calling it a ruler body shape, I don't think it's as much of a problem as being fat. A healthy lifestyle could still lead to body recomposition and/or body fat redistribution and it's always worth living a healthy lifestyle anyway. Even if that doesn't work, there are lots of "ruler shaped" lolitas who basically only have raised waist/empire waist/sack dresses in their wardrobe and effectively hide their lack of waist that way.
If a healthy weight ruler shaped lolita can find a corset that looks natural enough when worn under clothes they can stealth it, look good in skirts, "just waist" dresses and enjoy some of the side benefits (better posture, takes the pressure of layers of waist bands). I think I would try that if I was in that situation.