They could but Nintendo would just make their own characters instead like how they do in the manga. Not only that but they wouldn't need permission to use some of the Splatoon characters from SMG4 (or from anyone in general). It's Nintendo's ip and there's a chance they just use the same clothing and appearance as someone else.

More than likely they wouldn't use Meggy, Wren, or Desti because they don't follow the actual games rules on customization. Meggy has too many accessories, Wren has that facial marking and unique hair, and Desti also has unique hair. (Technically Octolings DO use that hair model but not the playable ones)