Yes, chromoly is a steel.

Some (very few) bmx riders ride aluminum frames, but more often flatlanders and racers than any other guys. I suppose it would work for some dirt if the guy is smooth and crazy weight conscious.

Carbon has next to no place in the bmx world. On bikes that take zero pounding (road bikes) or possibly on full suspension mountain bikes, you can get away with some carbon. There is a front hub that uses carbon in it for BMX. But, that's about it. Frames made of carbon get ripped apart in days, not years, like chromoly does. Carbon costs 10 times what the average rider can afford as well.

One of my friends worked at a local bike building warehouse and was amazed at how much road bikes sold for when there was so much less involved with them vs. bmx bikes.