It's less that shopping catalogs fell out of fashion. It's that the place that offered that particular one shut down. Like, he used to look through it to dream about all the nice things he'd once day be able to afford, and now even the place that offered them is gone. The childhood symbol of hope for a brighter, prosperous future crumbled. Dumb? Definitely. But it's almost like seeing the park you used to play in as a kid get bulldozed. It's more than just a park to you. Or when you first see your dad as a fallible human and not the invincible Superman you saw him as when you were a kid. It's reality, but the symbols of childhood hurt when they disappear.
The second 'joke' about the book is an old standard. Kids these days don't appreciate the things that came before. You're no longer with it. What you're with isn't it etc etc. Overall it's just filled with a sense that the world has moved on without you and a great sense of loss for everything you once held dear. The crumbling away of your childhood dreams as the reality of what you are and the world you live in settle in deeper.
Or it's just badly written jokes and my depressed ass is just projecting my hopelessness on to it.