Why is the toothpaste commercial that is playing on the subway particularly ironic when it is paired with the passage Montag is trying to memorize? The question is towards Fahrenheit 451, by Ray Bradbury.
The passage Montag is attempting to memorize comes from the Bible:
And why take ye thought for raiment? Consider the lilies of the field, how they grow; they toil not, neither do they spin
The speaker is asking why one is concerned about clothing; look at flowers. They don't do any work or create any clothing, and yet they are "clothed" beautifully. This passage speaks against worry over material possessions.
Montag's experience in the subway is ironic because the Denham's Dentifrice commercial is, fundamentally, about material possession. Additionally, it almost seems to be having an argument with Montag, speaking nearly in response to him in an attempt to distract or tempt him, ending with the "vomiting" of noise that seems to imply that the commercial wants to have the last word.
This is also a demonstration of the "sieve and sand" theme in action; not only is Montag inexperienced in this sort of mental exercise, but the nature of his world seems attuned to stamping out any thought by providing so much noise and distraction that he is incapable of doing it at all. In this environment, "considering the lilies" is impossible.