I just saved one sim from Grim after she collapsed from freezing. She was on the street in front of my sim's house and my sim convinced her to group up. It would take too long to walk back to my sim...
Reason is that it is not the looks o the clothes that determine its properties, but rather the function of the clothing slot where you put them.
So, if you put a winter coat into your Sims "Cool clothes" slot, s/he will wear it without any heat problems in the hottest summer. And if you equip your Sim with a Bikini in the warm clothes slot, s/he will run around with a Bikini in winter and won't get cold