it depends on the cause.
If you mouth-breathe because of allergies, a suboptimal airway due to bone structure, or something else, it's a cope.
If it became a habit during a problem and continues after the problem is fixed for some reason, then it might be legit.
But if you’re talking about the importance of mouth-breathing in teenage years for facial structure, then that is utterrly jestermaxed cope.
Breathing problems can impact the face in infancy/ early childhood
but by the time you're teen, development is largely complete, and the bones just settle into place during puberty, breathing won't have much influence.