From personal experience, I wear a plastic case all the time I use my MacBook (more than a year), it only made noise when I was playing games) for this period I just took it off and that's it) So no problem =)
If anything, I myself found this answer on one of the forums. True, it was given on October 3, 2012, most likely in the new models something related to this topic could have changed, so the question is still relevant.
"Correct if I am mistaken, but the heat removal system is carried out at the expense of alum . Due to this, the laptop works quieter. When glued with all sorts of films, a thermos is obtained. Maybe this is the reason for the banal overheating "