Whenever a Nintendo system (usually a home console) suffers from lacking third party support, one of the big go-to explanations for gamers and publishing execs is that owners of Nintendo platforms don't buy third party games, and only buy games that are released by Nintendo themselves. You saw this a lot during the Nintendo GameCube, Wii (to a degree), and Wii U generations. Nintendo versions of games often sold poorly compared to their PlayStation and Xbox releases, and even exclusives often under-performed.
But how true is this narrative? I can easily understand the phenomena with the GameCube. That was a console that pretty much catered exclusively to hardcore Nintendo fans throughout most of its life, creating an unhealthy environment for third party games on the system. There were exceptions, namely games that incorporated Nintendo IP into the GameCube versions like with Soul Calibur II, or games that were akin to Nintendo games such as Sonic. But most GameCube owners, bought it for Nintendo games.
With Wii U, the console itself was just very unappealing. Third parties making boneheaded decisions with releases (Mass Effect 3 at full price with missing DLC, while a trilogy on other consoles was the same price. LOL) didn't help either.
The Wii was somewhat of a mixed bag. Games that played well to the newbie/non-gamer-heavy user-base of the system found success on the system such as Just Dance and Boom Blox. But games that tried to be a "core gamer" experience were often met with disappointing sales. The slew of shovelware that the system became known for was also an issue.
These three systems all had easily explainable scenarios for mediocre third party sales on them. But what about Nintendo's pre-N64 consoles, the NES and Super NES? Those systems launched many of the biggest third party IPs and games. Or how about Nintendo's various handhelds. Franchises such as Ace Attorney, Monster Hunter, TWEWY, Professor Layton, Scribblenauts, Shantae, and others are heavily associated with Nintendo because of them.
And then there's the Nintendo Switch. The Switch has not only seen the biggest third party sales on a Nintendo system to-date, but Switch versions of games often sell almost as good as, if not, better than the PS and Xbox releases. Obviously Nintendo's own games for these consoles are the highest selling, but that's largely due to how Nintendo has historically integrated its hardware and software together over the years.
So the whole "Nintendo systems are only for Nintendo games" mantra is only really true if the console itself isn't very appealing to a mass audience. The GCN was a hardcore Nintendo fanboy console, while the Wii U was kind of bootycheeks. If a Nintendo system has good sales with an appealing hook, then games will sell well on it. First or third party