In addition to the problem of measly human power (mentioned in other comments), generators are only efficient when the magnets are quite close to the conductor

In addition to the problem of measly human power (mentioned in other comments), generators are only efficient when the magnets are quite close to the conductor so the metal pots/heating element would have to be held ~2mm from the rotating magnets. Generators succeed in this because they have a small metal housing that holds everything rigid. I don’t see how this could be practical for a large cooktop. Also the material of the pot/heating element (Al vs Nichrome) only changes the optimal gearing ratio for the bike wheel.