How do I get the Piranha Plant DLC?
Solution 1:
According to the official Super Smash Bros. site:
A download code* for the Piranha Plant DLC will be sent to the email associated with your Nintendo Account within 10 days after registering.
*Download code expires 11:59 p.m. PT on 6/30/2019.
Considering the game only released 2 days ago, you still need to be patient for at most 8 more days.