Actually I think they were going for more a gag reaction honestly Christopher considering the reaction we got from Castle. Also Beckett is the one who acknowledged Officer Hastings at the start as well. "Officer Hastings. What do we have?" or some such. It was set up from the start and paid off later on in the episode and the pay off wasn't Beckett's reaction, it was Castle's.
If they wanted a reaction from the audience this should have been episode 7. Before then, we see this cop at a couple of crime scenes, maybe drop something off by Beckett's desk in another. After three appearances people will be talking about her, wondering what her significance is.
Get the audience used to seeing her, and then you get a better payoff when she is introduced as the super-hero.