Martinet is indeed the exception here, and one of the most well known voice actors ever.
But there are plenty of other examples where my post holds true. The ghosts from Luigi's Mansion 3 are voiced by Next Level Games development and marketing staff, the voice for Yoshi is Nintendo composer Kazumi Totaka, Shy Guys are voiced by Nintendo of America localisation manager Nate Bihldorff, the WarioWare cast were voiced by Treehouse (localisation) staff until WarioWare Gold, Donkey Kong 64 had Rare's staff voicing characters (like composer Grant Kirkhope as DK himself), etc.
You're not wrong, but you're going a bit historical there with DK 64, which came out in the 90s. On top of that, British developers were notorious for picking employees for voice work.
Nintendo still uses internal staff for incidental stuff, but you can see the trend of professionalization. Tears of the Kingdom came out on the same day around the world with a large professional voice cast for every supported language. That's where Nintendo is going.
But there are plenty of other examples where my post holds true. The ghosts from Luigi's Mansion 3 are voiced by Next Level Games development and marketing staff, the voice for Yoshi is Nintendo composer Kazumi Totaka, Shy Guys are voiced by Nintendo of America localisation manager Nate Bihldorff, the WarioWare cast were voiced by Treehouse (localisation) staff until WarioWare Gold, Donkey Kong 64 had Rare's staff voicing characters (like composer Grant Kirkhope as DK himself), etc.