So based on the daily missions you can get 20 Miitomo points. The Mario outfit is 250 so assuming that's going to be the standard price for DX costumes it'll take you 13 days to get one just from these points. Depending on how regular the release of said costumes is that's not too bad.
Don't know if it's different by region, but there's a few more dailies that accounts for 35 points (click view all missions), in EU at least.
Is that the only way to get Zelda Picross?
at the bottom when you view all rewardswhere that flipnote 3d at
Am I obligated to buy the coin holder stuff or whatever that is? (50 points cost)
You have to go claim your coins on the My Nintendo page.I don't think I got coins for signing into Miiverse.