I don't think Miyamoto has much to do with Nintendo's PR or their official press releases, but I could be wrong. Miyamoto basically says every game he likes is a failure from a sales perspective (StarFox, Pikmin, etc.) and it's usually because he takes these projects as his baby. At least from my understanding of it. I don't think he's as good a source as Nintendo's IR reports themselves.
Miyamoto is still on the Board though, so he obviously has access to all of Nintendo's sales data. Even outside of Miyamoto, Nintendo didn't hesitate to confirm that Wii U and Super Mario Run failed to meet expectations (outright bombing in Wii U's case). People like to say ARMS or Labo are failures, but really Nintendo hasn't admitted anything of the sort. And again, Nintendo's publicly traded nature forces them to be honest about the commercial reception of a major release.