Is it common to drop "are" and "is" in informal English?
It depends a lot on what region of English you are asking about and what specific dialect. African American Vernacular (AAV), for example, follows this pattern frequently, even in semi-formal speech. However, some other dialects do not.
Personally, I am a fairly young person (mid 20s) raised and living in the northeast US, and I say something similar to these, but not exact. While I would never say "What you doing?" I commonly say and hear around me "Whatcha doing?" where the contraction "Whatcha" has absorbed the "What are you"