No, not necessarily. You see, Yoga is not contortion and nor is it gymnastics, although I can see why to the ordinary person some photos of Yoga positions could look similar !
It’s not very often that pictures are published of stiff yoga teachers- we just don’t look as photogenic as the really bendy ones! But that doesn’t mean we aren’t achieving as much personally.
Some contortionists are born with conditions that give them extra supple ligaments. Some gymnasts train to stretch theirs to extremes. Neither would be considered a spiritual practise- they confine themselves to the physical.
In Yoga we understand the body is our vessel: we have to therefore assist its health.
But we try to balance the body -not push it to extremes that could ultimately cause harm. So when I identify a weakness in a student I try to determine if it is a result of stiffness in the body or over-flexibility. I teach a lot of very bendy people in pain as a result of over working certain muscles in the body.
So trying to strengthen an area could actually mean learning to gradually reduce over-flexibility!