Yes, the hair dryers on the Spirit are weak, around 900 Watts of heat, instead of the 1800 Watts of heat your personal hair dryer puts out.
But the electrical receptacles available in ever cabin on the Spirit are only rated at 500 Watts. Plug your hair dryer in, and I guarantee you'll trip the circuit breaker. Then you'll have no power to dry and style your hair.
And the maintenance personnel will know why the cricuit breakr tripped. They'll come out and reset the circuit breaker, just don't expect very quick service, as they will have to reset other circuit breakers too. They are busy resetting circuit breakers on the Spirit every day.
You may get lucky and not trip the circuit breaker once or twice, but you will over the course of a week's cruise.
I highly recommend just using the weak hair dryer provided, especially if you have booked an inside cabin, if you don't want your stateroom to be in the dark.
There's only two 110 Volt receptacles in every cabin, one on the desk and one in the bathroom.
Both are on the same circit breaker.