In general, it's a pretty common thing to say, really. Any time there's a kid acting up and you're with a parental figure it tends to get used. Because all kids are frustrating at times, it's true. I think it's a bit funny used with Damian (which is why it made me think of GM's interview where he claimed Dick was like Damian), since Damian's definition of brat really is totally different than Dick's.