Some are, some aren't. There's lots of people here and lots of reasons to be stressed, so odds are in your favor of getting someone who doesn't want to deal with you if you stop them on the street.
Plus, after a while, you deal with so many scammers/homeless/drunks on the street, you learn how to brush them off and keep walking. Most times, you can tell in someone's eyes when they're approaching you what they want. I'm not sure Sasha Baron Cohen is such a good actor to not give himself away to a sharp New Yorker before he's even opened his mouth. Of course, I'll give old people and clueless tourists directions or whatever if they need help most of the time.
The only people I really get pissed at are those that cop an attitude when they ask me something and I can't hear them because I have my headphones in (canalphones). Sometimes I pretend they're hearing aids and feign sign language at them just to confuse them.