Is or should be their main editorial goal to exactly mirror the political affiliations of americans? Is that the issue Berliner was raising in his essay?

I didn't read it that way, and I do find this relevant to the points he was making, which were much more about journalistic practice and ethics than about the demographics of listeners per se.

Part of the problem with this stat, which I saw someone point out on Twitter, is that conservative demographics have changed since 2011. College-educated white voters, especially women, have shifted significantly toward the Democratic party during the Trump years, and that was probably the biggest listener demographic for NPR.

So NPR listeners in 2011 and 2023 could be the exact same people and the % of conservative listeners would have gone down. (That said, I suspect this isn’t the only explanation - NPR content has gotten more ideologically left during that timeframe too)

To what degree is this shift due to NPR's own actions vs. the pressure of the former president?

I would guess that any news source that is not specifically pro-Trump has bled conservative viewers/listeners/readers in the last 8 years.