Some fun stats to start with:
If you are from a 1st world nation you will spend an estimated 96% of your life indoors or in a car. If you dont buy that take notes on your daily routine. If you are from a 1st world nation chances are you are from a part of the world that has winters. If you have winters you must have heaters if you convert your body heat into electricity you will radiate less body heat. Less body heat means running a heater more. Running a heater more means more fuel is used.

Now, you are going to say that human body heat is to low to worry about anyway. If you say this then I ask you why you are trying to charge your phone with it.

You might point out to me that people run ACs in the summer time so this will be a moneysaver. Only problem is that the efficiency of any process that converts thermal energy into another form of energy is strictly dependent on temperature differences. So, in a nutshell in the winter it will run the best but that is the time you do not want it to run, in the summer it will run badly even through that is the time you want it to run.

What about outside? I guess it could have some use outside if say you were jogging. I would still wonder through if you would really want this fabric pressed close to your skin.