In another question I asked, I confirmed that you can encounter the Weather Genies again without doing the Pokeleague, even if you faint them or run away. Does this also work with the Sinnoh Dragons? Or would I have to do the Pokeleague again to respawn them?
(Can't test this myself because I don't exactly have a spare Lake Trio or Palkia just chilling in my PC)
To encounter them again after fainting them, you will have to defeat the Elite 4.