It's both a few deg cooler in Chiang Mai (NW) and is slightly lower in humidity compared to BKK. It does feel much better up there. I haven't been to Isaan (NE) so I can't compare there.
When my daughter was living in Dubai she said newcomers always got Dubai 'flu' from the air conditioning there. Air conditioning is essential but it seems it takes a bit of getting used to, the 'flu' ( presumably some sort of respiratory problem) is quite unpleasant.