>>11961846Meloetta isn't OU because in it's Aria form it's pretty much a slow Latias or Latios with a worse primary STAB and less defensive bulk
Pirouette form is too hard to get out and maintain a sweep with, meanwhile pokemon like Terrakion or Keldeo can simply show up and smack your shit. Plus Pirouette necessitates Relic Song, and to do decent damage with it you need to invest in special attack which directly goes against the Min-maxing concept of the game
