Helen Parr aka. Elastigirl has the power to bend and change her shape, so is she choosing that shape or is that just the way she is?
Aside from the reason stated in the other answer of her needing force/to concentrate to be stretching we can also infer it’s her natural state from another reason. Every time we see her unstretch she returns to her normal state by snapping back into it. To me this indicates that the usual state we see her in is her natural form. We see it happen a couple of times in the scene below:
The image provided with the question looks to supply the answer.
In that scene, Elastigirl appears not entirely happy with her shape in the new skintight costume. (It's been a few years and three children since her last outing in a hero suit!)
If Helen Parr could adjust her figure at whim, that scene would show her butt inflating and deflating until she found the size that pleased her.
Superpowers usually require some level of focus and concentration to use. You have to decide exactly how much strength you want to use or you have to decide to "kick in" your super speed. The concept is similar to elastic powers.
Notice the keyword "elastic" that says what she is doing is stretching. So that requires her to decide how much and what she stretches. If this weren't her natural shape, we would be seeing her change back to what is her natural shape occasionally, which we don't.
So the conclusion is that, yes that is her natural shape.