Might make me a little nervous. Not much. I have a lot of acquaintances that act like morons pretty frequently, but are nowhere near as dumb or careless as they act.
Case in point: A guy I know, kind of a friend, kind of a friend-of-a-friend, has gone shooting with us a few times. He never did the "gangsta" thing, holding the gun sideways, as I recall. But it's the kind of thing he would do, to be funny. He isn't unsafe, though, and he learns fast and is serious when actually shooting.
If he had done that, I would probably have laughed, and then said something like, "Yeah, man. Real Gangsta. Now here's how you hold it if you actually want to hit something."
As long as he's joking, which he almost certainly was, it wouldn't bother me. I'll be honest: I've shot handguns "Gangsta style" before, just to see what the fuss was about. Not at a range, mind you, but still. Just joking around doesn't make someone a careless shooter, unless that someone is joking around at the range.