I tend to read the theme of this book along rather specific lines: participating in popular fads is equivalent to living in a dream world.
The young lovers are inauthentic, just as Don Quixote is inauthentic. His lack of connection to reality is just more obvious, but, in some ways, less toxic.
Seen this way, the novel's theme is certainly relevant today.
I do think that Don Quixote can be applied to modern culture. The tone is humorous, but the theme is serious. I have always interpreted it as this: You need to be yourself, and go after your dreams, even if they are unreachable.