As a pulmonary & critical care physician (who treats COVID-19 patients in an ICU setting) and pilot, the purpose of a non-N95 mask is to minimize transmission of the virus. It is not 100% perfect but it is 6-fold greater than no mask at all. This is not some musings of the media, but based on scientific data. Although the airline safety briefings state to remove one’s mask before placing an O2 mask in the event of depressurization, placing an O2 mask over one’s mask would provide adequate oxygenation. If you need to prove it to yourself, you can do so if you have an oxygen source in your plane and a pulse oximeter. Fly at 9000 ft MSL or higher and with your cloth/paper mask on, measure your O2 saturation with and without wearing your O2 mask.
I’m more concerned that the passenger oxygen source on commercial airlines might NOT be operational when needed (I’ve had that experience once).