If you're looking at Miis from a site that includes QR codes alongside each one (say from a site like miicharacters.com) then you can scan in any Mii character you want to download with the scan QR code option. Now the Wii U only has a front facing camera so this can be a little awkward, but still totally doable without having to create each Mii one-by-one. Or, if you have a 3DS (or 2DS), you can do the same thing there but with the back facing camera which is far less awkward, and then on the Wii U, you can wirelessly transfer over all the Miis you put on your 3DS. I know Miitomo on smartphones used to let you scan Miis too, but I'm not sure if you could transfer those Miis to the Wii U or not, and even if you can, I'm not sure if Miitomo is even able to be used anymore, pretty sure it's completely dead; but potentially that might be a way to transfer Miis as well to Wii U after scanning them in. So you have a few routes you could go here, they all hinge on scanning QR codes though.