The reason for experiencing stiffness is not the same thing as being actually stiff. Lack of recovery, not enough sleep, high stress load, inflammation or a flu, are some reasons for feeling stiffness,
If your indicator of feeling stiff is the reason why you add yoga, mobility, stretching (or whatever you like to call it) it might not be relevant and appropriate all the time. It might be lack of strength or endurance, not enough sleep or experience with pain that’s causing the stiffness. What also influences our subjective experience of bodily state might be the people and environment around you...
Is stiffness a feeling or a belief?
Do you verify (validate?) range of motion from an external view or is it the internal sensation?
And if you need to categorise yourself as stiff, when do you think not being stiff will happen?