Throughout David Bowie's career, he's been well known for the intellectual depth of his songs, and has been called everything from a postmodern
artist to an "auteur."
"Life On Mars?" is very complicated thematically, and much of that depth comes from various shifts in the lyrics that, while subtle, offer a lot to interpret.
Take the narrative shift from the first pre-chorus to the second pre-chorus. Here's the first pre-chorus: But the film is a sadd'ning bore
For she's lived it ten times or more
She could spit in the eyes of fools
As they ask her to focus on
Now here's the second pre-chorus. But the film is a sadd'ning bore
'Cause I wrote it ten times or more
It's about to be writ again
As I ask you to focus on
Notice how in the first pre-chorus it's assumed that the makers of the movie she's imagining watching are the "fools" who've given her the same thing "ten times or more." In the second pre-chorus we shift to the first-person; now it's the narrative voice—Bowie himself—who "wrote it ten times or more" and is about to write it again. What does this shift offer? We have Bowie indicting himself as a writer who produces the same stuff over and over again, where earlier Bowie called authors who do that "fools." How does Bowie get away with calling himself a fool?
Another stimulating line in "Life On Mars?" is the chorus ending: "Is there life on Mars?" What does it mean—and who's speaking, and in what way? A way of interpreting the lyric is that the "girl with the mousy hair"—whom Bowie has called "anomic" (alienated and socially disoriented)—can't find anything to connect with in the torrent of images she encounters daily on Earth. "Is there life on Mars?" then becomes an escapist plea and a hope that there's life somewhere else in the universe that she can connect with, at least.
Or the line could be rhetorical. The chorus is a sort of call and response between the images of the "silver screen" and the girl's reactions ("Look at those cavemen go / It's the freakiest show"). The question may be identifying Earth itself as a kind of Martian environment in the sense that our cultural tropes are so alien to her—not to mention dead in the sense that they're overused and uninteresting to the girl.
Another option, perhaps syntactically more solid that those other two interpretations, is that the phrase "Is there life on mars?" is the object of the sentence, Take a look at the lawman
Beating up the wrong guy
Wonder if he'll ever know
He's in the best selling show
Is there life on Mars?
…Meaning that the "best selling show" is entitled "Is there life on Mars?" In this sense the recycling of old ideas in pop culture becomes a sort of spectacle of the failure for authors/producers to find meaning ("life") in a barren waste of overused images ("Mars").