More like the older generation Japanese would still dress up when going to J (personal observation) and F while the younger one tends be a little bit more casual (but not to the extent of shorts or singlets).

While Malaysian (cant speak for other Asians) who boarded JL J and Y class are way more casual when compared to Japanese pax except for few who are heading straight to office once arrived in NRT at 6am

End of the day I think person manners towards other pax and crew are more important that what are you wearing for the flight.