Insulation is probably partly cost, and partly weight, which can make a difference to mpg. And as you say insulation over the HV battery may cause it to overheat , or use its battery cooling fans more often, which increases fuel consumption and may add fan noise. (some say they can hear this- I've never noticed it )
If you do your own insulation you'd need to be careful not to block or impede any air flow.
I only have fitted floor mats , which may help a bit.
Thin guage bodywork is increasingly common. It saves a lot of weight on an entire car. Manufacturers may claim they use higher grade steel than previously , and carefully calculated by computers. Some cars only feel stronger because their styling incorporates more compound curves that makes it feel more rigid, a bit like corrugated iron roofing.
Hondas bodywork and rustproofing is not world leading. Maybe because the Japanese government have a policy for the japanese market that favours exporting the car to other countries when it reaches 3 years old and buying a new japanese car at a cheap price.
But all cars tend to reach their end of life either because of its bodywork, or its mechanicals . You often see images of perfectly decent looking cars going into the crusher. Honda owners may lament that the car still had perfect mechanicals , other makes will lament that the body was perfect and the mechanics let it down. And a car that was class leading in both areas may have been a heavy ,gas guzzling, slug all its life.
