A Teletubbies game would be rated E for EVERYONE. Does the game appeal to adults?

Poor example, Teletubbies is aimed directly at children, Splatoon/Mario/Zelda are not. I think you'd find most of their players would be adults.

I'm sure you wish I was mad. Fortunately for me, I don't give a shit. 

No shit most shooters are M-rated. Which is why it's so interesting that one isn't. Most of Nintendo's demographics consist of adults; I hate to break it to you. And E means for Everyone. It's not K for Kids. I've never seen more children play a game than Call of Duty, and that is supposed to be a mature game. Irony.

I guess Star Wars and Citizen Kane are little children's films with no appeal beyond kids and teens (Kane is PG)?  Since, ya know, they don't have that R rating?  Appropriateness ratings are purely gates to advise against exposing certain age groups to certain potentially objectionable elements, they don't denote a level of appeal or limit the target demographic.  They certainly don't denote what is genuinely mature and what is not.  A Teen game can be more mature than a Mature game.  Heck, a E of E10 game can be more mature than a Mature game.  Because E, T, and M are appropriateness ratings, not content quality evaluations.

My guess is that the game is heavily customizable with dlc. 

BASS: THE MOVIE A tail of fishing is rated E for everyone. Does this appeal to adults? Yes. It can be sold to EVERYONE. Yeeesh 

The way that Nintendo advertise their games contradicts what you are saying.

https://m.youtube.com/watch?v=mn2nqR3f7JM

Not that you would know who you are playing with on Splatoon, i mean....No Voice Chat.

The mighty silence of adulthood!

You're grasping at straws. That doesn't invalidate any of my points.

Their advertising does not reflect their actual consumer base, which is why it's ineffective. 

CS Lewis said it best:

“Critics who treat 'adult' as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.”