I hate the torpedo looking helmets they have for mountain biking. Is the BMX style ok? The half mullet style like you see at the x-games.

Yes it is completely fine, as has been mentioned the reason those "torpedo" style helmets are used is simply because they are more ventalated. In actual fact, the bucket style helmets worn by BMX riders offer a much higher degree of protection because they are A. Harder and designed to take large hits, and B. They cover more area of your head. I have used the bucket style helmets for some time now and find that the difference in how hot my head gets is more effected from how im riding than the helmet im wearing. So if you feel you would rather wear a bucket helmet then feel free to do so, just make sure it fits correctly and all like you would do with any helmet. Good luck bro