If we are talking about the main series, then the first Gen8 DLC, the Isle of whatever, is the only time I thought main series content was terrible. And by that I mean complete underhated garbage. I cannot stress how much I despise that stupid DLC and how worried I was back then hoping that Gen9's DLC won't be as atrocious as SnS's first one.
Awful plot, most of the new characters were boring, the only feeling I felt every time I played it was "Wait, why the HELL did they do that?" or "Why am I playing this disasterpiece of a DLC for the fourth time". And I played it at least four times HOPING that I would eventually see it as good but it only got worse each time. I have played original SnS without the DLC and I find it completely playable without it, for the record.
Thanks to that DLC, 2020 is an abhorrent year for gaming.
That's literally the only time I thought main series content was irredeemable, and since then no games or even DLC quite as bad as that. So as long as it's not as bad as Isle of Crapass, I'm fine with more new games.