Thermal efficiency is purely a measurement of how much of the energy you put in, goes to actual work, and how much is wasted through heat.

Mass only plays a part in that thermal efficiency might change depending on the load the work is performed on.

I can't think of a single engine that have better thermal efficiency than an electric one. (Not taking into account how the electricity was produced)

You're right about it being a moot point. There are far more important aspects than simply thermal efficiency. I just wanted to set the record straight. Because saying humans have better thermal efficiency than cars is just not true. Not even close.

We evolved sweat for a reason. Our thermal efficiency is so bad we had to develop external cooling or we would overheat.