I read that inflated jackets keep us warm because air is a poor conductor of heat so the jacket does not conduct the heat from your body to the outside environment. If that is the case, then why doesn't the same inflated jacket keep us cool during summer by not conducting the external heat into your body? Instead we feel hotter if we wear the jacket.
I believe the reason is in the summer the main way the body keeps cool is by means of perspiration and evaporation. This requires air flow across the surface of the skin. Wearing loose clothing allows air to flow past the skin carrying heat away from the body. A snug jacket, even if filled with air, prevents evaporative cooling since it prevents air from circulating on the skin.
Bottom line is in the winter you can alway wear more layers of clothing to prevent heat loss. In the summer the less clothes the better to allow evaporative cooling. Of course you still need to protect the skin against sunburn, so loose fitting clothing is still needed.
Hope this helps.