Gaming

Nintendo is a "family friendly" brand before all else and really only cares about the experience of children playing their games and adults buying their games for children to play. They count on their core IPs to draw in those kids as adults, but don't put much effort in catering to an adult audience. They put more effort in with the Switch (game store with more adult oriented games), but still minimal effort - their original properties are family friendly.

They see other people using their IP as diluting their brand value rather than promoting it. They think their characters are what makes people nostalgic for their games and drives brand value. So they want you to only be able to see your "favorite Nintendo characters" from Nintendo official sources and have complete control over that experience.

I think they're wrong about most of that. The characters are, for the most part, pretty generic and simple. What people like about Nintendo is that the games are accessible, they played when they were kids, and they were often introduced to those games by parents or older siblings. There's a social context to Nintendo games that is unique and nostalgic. They're often some of the first games you play as a kid, and they're the first games you think of when you want to introduce your own kids/nieces & nephews, etc. to gaming. I don't think that unofficial Super Smash Bros tournaments or Gary's Mod having fan-made Mario models in it dilutes that in the slightest but Nintendo does drive away adults who are the primary drivers of the Nintendo brand's popularity (as they are the purchasers). Once it's these young adults' turn to share Nintendo games with the next generation, I think Nintendo's litigiousness will hurt them because it will have driven many of these people away.