Why would someone breathe through their mouth if there's no nasal congestion. I'm pretty sure that you'd have to be cognitively impaired to begin with.
Top entry for 'mouthbreather' on Urban Dictionary is from 2017: https://www.urbandictionary.com/define.php?term=Mouthbreathe...
1. Some people's nasal passages are partially or fully clogged.
2. Even when fully open, there is not enough airflow through the nose to facilitate anything more strenuous than light walking.