I don't mean when guys/girls open their mouth wide in pictures like a sign of shock (which is pretty unflattering IMO).

Mouth breathing really fucks up facial development for kids and could result in long term damage to their teeth and jaw recession. Seeing their celebrity idols opening their mouth in pictures could indirectly help them develop bad habits of having their mouth open and result in poor tongue posture. Of course they may not actually breathe from their mouth but it's most likely that people with their mouth open mean their tongue will not be resting on the roof of their mouth, making the tongue more susceptible to sliding back into the airway when they lie down. This can develop into sleep apnea, which is a direct cause of mouth breathing and other health complications. I get that "it's just the beauty standard" and it definitely isn't something that can change over night, but their influence definitely plays a part in current trends amongst youth today.

Edit: Longer explanation