Let's try to ignore how "all time" and "past 10 years" in the same phrase is tabloid-level sensationalism, or how 10 years mean different outlets, reviewers, thus it's not so reliable for comparisons across long periods. Let's focus on this year's output.
This has been one of the lower years for Nintendo, I don't think anyone can deny that. It's a year where Nintendo released way too many spin-offs in a short amount of time, with some of them being games of poor quality (Amiibo Party and Mario Tennis: Ultra Light as posterchilds), and a lower amount of bigger titles, which creates a context that enhances the amount of spin-offs (especially for the Holidays). It's also a year where they released titles like Splatoon (which is still evolving), Mario Maker, S.T.E.A.M., Majora's Mask 3D, Xenoblade 3D, Yoshi, Xenoblade is coming. Plus, continued quality support for Mario Kart and Smash Bros (not including expansions in comparisons of mere games' output, just saying that these were other kind of developments Nintendo worked on, and is still working on, this year). So, we had good, if not great games, but they were less than other years, while titles with lower quality were more.
Still, bad releases like Amiibo Festival or Mario Tennis can't be excused by Nintendo focusing almost completely on NX internally: it's a known factor that influenced this year's releases, but those two games (especially Mario Tennis' lightness, from a certain point of view) are plain bad even when considering resources elsewhere.