Usually it's half Nintendo, half third parties. So that narrative is false. However, there are more third party games than first party. In that sense, it makes sense why some people think third party games should sell more. The ratio shouldn't be 50:50. This doesn't mean software aren't selling tho. The Wii and DS sold close to 1 billion, and the Switch has passed 1 billion. It's just that Nintendo first party games are really popular.
The problem is simple. Nintendo hardware are weaker compared to other popular hardware. So modern AAA games aren't going to sell well, third party needs more AA games. Most third party software sold on a Nintendo platform are AA games for that reason. Third parties are way too obsessed with making AAA games. We've been hearing how AAA games are not sustainable, why not make more low budget games. Like if Square Enix has the budget to make Forspoken, make a game for the Switch that isn't from Team Asasno or stuff that already exists. Third parties lacks the mindset to do something new, they don't innovate. Indies are trying their best, but even indie games most are just stuff that already exists. There's also a limit to how much budget they have. Nintendo platforms should be a place to put new ideas into form, I get that implementing motion control and stuff are difficult, but those are significantly more unique than making another generic AAA games to the market. Same with VR, there should be more VR games. Nintendo hardware has so much potential for these third parties, but for some reason they just don't care. Perhaps Nintendo platform and the Switch 2 can potentially benefit from AAA becoming unsustainable.