This is unusual, since newborn would not usually use his mouth for breathing. I suspect that the baby is crying with his mouth opening. It is important to observe for the baby's feeding, and to see whether he is gaining weight normally (about 20-30 gm per day).

Different organs would mature at a different rate, even for a full term baby.
I fully understand the worry of a inexperience mom, do consult a senior such as grandmother, or consult a paediatrician. Maternal Child Health Centre staff can be helpful too.