Because the sentence is not about Lowell himself but about something Lowell did or made. This is why you need to refer to it using “Du”.

As this phrase is a shortcut for “C’est du Lowell to craché” where “C’est” indicates you are talking about something Lowell did (here probably some kind of stylistic writing effect), the shortened version needs to keep this “Du” as a clue.

Furthermore we sometimes (less often though) say things like : “Ça, c’est Lowell tout craché !” ; so without “Du” but keeping “C’est”. I can’t tell you why exactly we sometimes say it one way, sometimes the other, and I am not even sure this alternative is grammatically correct ; but I happen to say it myself or hearing it.

As Ctouw has written du here represents something Lowell did or made.

C’est du Lowell tout craché.

• If we’re talking here of a poem by Lowell it means this poem is typical of Lowell’s writing.

  We could rephrase it this way :

  C’est représentatif de l’écriture de Lowell.
  C’est représentatif de ce que Lowell écrit d’habitude.

• If we’re talking about my brother Lowell – famous for getting into mischief – it means this new prank is typical of Lowell’s behaviour.
  We could rephrase it:

  C’est représentatif de ce que fait Lowell d’habitude.
  C’est représentatif des bêtises habituelles de Lowell.

Du, de/des, de ce don’t represent a person and it wouldn’t change if it were a girl.

C’est du Sophie tout craché.

But careful if the person’s name starts with a vowel sound.

C’est de l’Hugo tout craché.

C’est de l’Annie tout craché.

• You don’t “need to place du” here but not having it could be ambiguous if out of context.

  C’est Lowell tout craché.

  can either refer to what Lowell did, c’ representing something, but it could also represent someone, being rephrased as:

  C’est le portrait craché de Lowell.