That's a good question! It is possible, in some cases, to use the neuter form of adjectives even though the noun isn't neuter, usually when you describe food as good, healthy, unhealthy etc. See:
2 Ny grammatikk - NTNU
But "Billig mat er usunn" is also correct, and would work just as well in this sentence.
By the way, the use of "tenker" in this sentence is traditionally seen as incorrect, but it has become widespread -- probably because of influence from English. I would prefer "tror", "synes" or "mener", depending on the context. See:
Å tenke i tide og utide
I was also surprised to see "halvfabrikat". I don't think I have ever seen this form of the word before, only "halvfabrikata". But when I checked my dictionary, I found that "halvfabrikata" is the plural definite form of this noun, while "halvfabrikata is plural indefinite.