It's not on the Nintendo Switch Bestsellers list. If it was at some point, it's definitely not anymore. You could make an argument that the reason this isn't the case anymore is because over the years since DX's release, there have been newer, more exciting games, but I think it goes to show the niche popularity that Pokken Tournament has. Mario Kart 8 Deluxe has held on pretty strong for about 2 years now, so it stands to reason that the long-term appeal of Pokken Tournament DX is probably in question.
This is all to say that I'm still very much unsure whether the games will see some sort of sequel in the future. Anything can happen, so I'm hesitant to say that they
won't, but it seems unlikely.