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One consideration is getting a uniform distribution of heat. That would require either an extensive array of individual resistors or else something like a zig-zagged length of heating element.
You may be surprised at how much power is needed to de-fog the mirror in a steamed-up bathroom.
Don't know where you live, but local regs may prevent the use of portable mains-powered devices in bathrooms, on safety grounds.
Since the dew-point depends on humidity level, triggering the heater at a set temperature may not be feasible.
Heating power (averaged) could be done with oscillator-based PWM.