well yeah, its bad. its kind of mixed though, i think the pros of using a blowdryer way outweigh the cons so ill tell you here:
pros- my hair feels lighter, softer, and never gets tangled or frizzy if i blowdry.
cons- if you blowdry wrong, it will get frizzy. and my sisters say it kills your hair but hair is already dead. and it may dull the color if you have color treated hair but for that you should just deep condition once a week or use some amazing shampoo and conditioner by pureology for instance.
i use my blowdryer about 5-7 days a week. meaning i use it everyday and at the least, 5 days. my hair looks best when i blowdry, its that simple.