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.