Category term for "polite", and "offensive"

I'm not aware of any word that's a good fit in the sentence as shown; more possibilities may exist if agnostic is replaced by neutral or free. However, consider the word value:

It's not specifically polite, but it's not offensive either; it's value-agnostic.

Note, value is being used above to represent rules, principles, conventions and judgement regarding manners, etiquette, civility, politesse, habits and behavior, rather than in some other sense.

Two possible rewordings that avoid the need for a word as requested include the following:

It's neither specifically polite, nor offensive either; it's neutral.
It's not specifically polite, nor offensive; it is amoral.

How about mores:

A set of moral norms or customs derived from generally accepted practices. Mores derive from the established practices of a society rather than its written laws.

So you'd have: "It's not specifically polite, but it's not offensive either; it's mores-agnostic."

It is a matter of courtesy, and so is courtesy-agnostic if you would.

The problem here is that being polite is more or less the normal state of affairs, and being rude is conspicuous failure to be polite. The OP's proposed utterance is probably going to be awkward no matter what one chooses, just as the following would be a bit difficult to interpret:

She's not pretty, and she's not plain; she's looks neutral.

On the other hand, try out the following, which picks out two adjectives occupying non-contiguous ends of the attractiveness continuum. (It sounds easier to interpret):

She's not gorgeous, and she's not ugly; she's looks neutral.

To get what the OP wants, let's make a similar adjustment:

It's not prim, and it's not offensive; it's politeness-neutral.