First off, generalisation isn't cool.
I've posted on Miiverse, quite a bit actually. You probably have too. Not to mention all of the great artists on there, like
BluePikmin11
often shows us. Guess that makes us all idiots because of a few kids who happen to hang out there?
Second, I could very easily say the same thing about Smashboards users not being "evidence of anything" (personally, I've seen way greater stupidity on Smashboards than on Miiverse). Doesn't make it true, though. Again, generalisation isn't cool.
Thirdly,
the IQ level of a demographic is irrelevant. If they show support for a product (in this case, Rosalina being playable in Smash), then that's a GUARANTEED audience they can please. We don't have any other official sources we can reference for this, Sakurai's Miiverse posts are the only solid statistics we can look at for how well specific elements in the game were received. And they tell us that Mario, Sonic, Mega Man, and Pac-Man being collectively in the game together (but not alone), was what people were most excited about this game for, period. And Rosalina & Luma ALONE were right on their tail. That's nothing to scoff at.