I just randomly found this a second ago! All you have to do is scroll down to underneath the daily poll section, and you should see Cappy float down a bit directly underneath. Click Cappy and you're good to go!

Visit the site here

UPDATE - Okay, there are multiple hidden My Nintendo missions on the site. Let's hunt them down! Reader Akamu found another one on the tournaments page. Click the image of Biff in the ARMS section by commentators, and you'll get another secret!

The first one we found was mission two, the second one Akamu found was mission 3. At the very least, we're missing one more mission!

UPDATE 2 - Reader One-of-Three found the third mission, which is actually marked as the first. You can find info on it in his tweet below. Now, either we've found them all and there's only three missions, or there's still more hiding!