My daughter’s friend does the peace sign thing too.
Those links do not even attempt to explain it.
And surely no-one who ever saw such a photo of themselves (and kids these days surely see very many) could really think the lip thing looks sexy. The “duckface” site’s explanation does not make sense.
I have always assumed it is more or less the opposite: that out of shyness or lack of confidence they make these faces and gestures to say “Hey, don’t mock me, I am not trying to look sexy or cool. I am just goofing about.” However, why these particular forms of goofiness have caught on, I have no idea.