That's how it appears to be to me. Within the DSM there are frequent mentions of the need to take backgrounds into account when performing a diagnosis, since "normal" varies by context.

To provide a couple examples...

"In addition, cultural and religious background must be taken into account when considering whether beliefs are delusional."

"Cultural and socioeconomic factors must be considered, particularly when the individual and the clinician do not share the same cultural and economic background. Ideas that appear to be delusional in one culture (e.g., witchcraft) may be commonly held in another. There is also some evidence in the literature for the overdiagnosis of schizophrenia compared with schizoaffective disorder in African American and Hispanic populations, so care must be taken to ensure a culturally appropriate evaluation that includes both psychotic and affective symptoms."

Those kinds of mentions are found all throughout the DSM, and they seem so suggest that even if two people hold the same characteristics, only one of them could be considered to be showing signs of mental illness if the other lives in a culture where those characteristics are deemed to be normal.

That's what I was trying to illustrate with the ancient Roman example. If we view ancient Rome through the lens of the modern world it becomes a horrifying empire where it's citizens openly engaged in behavior that we cannot fathom any sane person doing. Yet if we observe their behavior within the culture they lived (like the DSM mentions doing) rather than by today's standards, I'm not sure we could call them mentally ill within that context.