Bobbi, it's fulfilling one of the things that comedy can do really well ... making you see how ridiculous and inappropriate something is by looking at it from the other side. Some people don't "get" this type of comedy and it can be difficult when taken to extremes, e.g. by Jimmy Carr or Frankie Boyle. But Ed Byrne takes it just far enough, IMO, especially with this "gorgeous" joke.
If an 11 year old girl has "Gorgeous" emblazoned across her backside, what's the point? What are adults supposed to make of it? What about 14 year old boys? What about other 11 year old girls? Who exactly is such a slogan aimed at?
Ed Byrne exposes it for how stupid it truly is by making a comment that would be appropriate if the slogan itself was appropriate - since the slogan wasn't, the comment wasn't ...
If Ed Byrne had been making a joke about the Justin Bieber t-shirts it might have gone something like this (with apologies to Ed Byrne) ...
I was out shopping with my young nephew the other day. He's a lovely lad but owing to his extreme shyness he's been having trouble with the ladies. Anyway, we saw an 8 year old girl wearing a t-Shirt that said "Justin Bieber I want your Baby" across it. And, although hopelessly optimistic, she seemed quite a tasty little minx, so I said to her "Ah, you're never going to get Justin, Too much competition, and he lives in another country! But here's my 16 year old nephew. He's willing if you are..."