Iambic pentameter is stylized, but one reason it was popular among poets (Shakespeare obviously the best known among them) is that it’s not that far away from normal speech. If this was a verbal conversation and I said that last sentence to you, the stresses would have gone like this:
It’s NOT that FAR aWAY from NORmal SPEECH. And I didn’t even plan the line to be that way. Iambic pentameter is broken up into lines of five iambs (or ‘feet’), and Lose Yourself really isn’t. So by a strict answer, no, it wouldn’t be. But who wants to be strict?
In reality, parts of it do fall into the unstress-stress rhythm, as all speech will from time to time.
“You better lose yourself in the music, the moment.”
That sounds pretty natural (except for ‘the music’), and it’s pretty much how Eminem says it. By the way, not all of the stresses are equal, so it’d be acceptable that, say, “lose” would be emphasized more than “the.”
However, it crashes and burns in other places.
“To seize everything you ever wanted
One moment”
If it was an attempt at iambic, it’d be pretty awful. You normally stress the first syllable of “every,” not the second. Ditto for “ever” and “wanted.” There’s no reason to stress “you” in this context either, really. (See, that’s the other trick. Iambic pentameter has to be written so that the pattern of stresses doesn’t just mirror speech - it has to make sense, and convey the emotions of the characters and the meaning of the scene. Fitting into an off/on pattern isn’t hard, it’s the latter part that’s the real trick.)
So I’m gonna say no, “Lose Yourself” is not in iambic pentameter, and I doubt it was Eminem’s intention to make it so - again, the bits that fit the rhythm are there because that’s how people talk. If it’s gonna be in iambic, it’s got to be ALL that way, and it isn’t.