3rd party characters should be judged only by how much history they have with Nintendo any how popular they are with the Nintendo community. Smash is about representing Nintendo history after all. Cloud and Joker didn't used to be very popular with the Nintendo community, but the Switch eShop version of FF7 sold very well and Joker now has at least 3 games on Nintendo platforms, so they are popular with the community now.
As for World of light, I agree that it can become repetitive, but I still think it's much better than Subspace.