Yes, "get wasted" is informal. There are hundreds of similar expressions... get maggoted, get sloshed, get pissed, get rotten, get flogged, get pickled, get trashed...
I wouldn't think too much about the precise meaning of "wasted". I guess it refers to the messy state that people achieve when they are too drunk. As though you've reduced yourself to the value and usefulness of a piece of rubbish/waste. "Wasting your life/time/day" doesn't equate to "getting wasted".