Thermocouples would probably be more efficient than your body’s natural radiation, assuming your skin could take the excess heat.
It can’t be warmer than your body or no heat would flow into the engine, otherwise it would require energy to draw the heat into the engine, in which case you’re taking energy rather than generating it.
In terms of a heat engine, the thermocouples aren’t the engine. Your body is the engine and the heat radiating off it naturally is the waste energy. Thermocouples under the skin would only improve the efficiency of your own body as a heat engine, since some of your waste heat would become energy.
In fact, adding this as a separate bionic at all is silly. We should just make the internal climate control CBM generate electricity instead of using it up when the ambient temperature is lower than your internal temperature (which would be most of the time).
EDIT: Turns out, seems the human body is around 20% efficient while most thermocouples are only 5-8%, or up to 15% for some new experimental ones. Not sure if it would insulate you heavily and cook you alive or just only produce a miniscule amount of energy…