Is the English “because noun” an instance of grammaticalization?

If only.

No, it’s a novel elliptic construction (drop the copula and subject, by default, including an expletive subject: because reasons = because there are reasons). The form Wellington Mendes reports, because wow, is a straightforward analogy.

But the function, meaning, and phonetic content of because has not changed: it’s still a conjunction. Its scope has changed, from a clause to a small clause (cf. paint it black, kill you dead—I don’t believe it is a preposition here). But that’s the only change characteristic of a grammaticalisation, and it’s not enough in my book to do it.