You may use a dimmer. Electrically this is fine. The real problem with dimmers is they do not make the heater get cooler as the air temp rises; the heater gets warmer as the ambient air temp gets higher for the same dimmer setting. This means you have to adjust the dimmer setting often.
A better method is to use a thermostat. This way, you have control from overtemp conditions.
Porportional controllers like Helix and Big Apple Herp Power are great devices but cost $100 or a little more. You can use a cheap analog T-stat for $30.00. Bean Farm sells them as does many other places. You'll need to fix the probe of the contoller to the heat pad or lay it on the heated area.
Good luck.
-----
Mark G
Animals:
A few Rosy boas, mountain kings, Cal kings and Children's pythons.
Caging:
Visionariums, Herpcages.com, Precision, 5 gal tanks for babies.