One should not underestimate the possibility of Nintendo wanting something new for their biggest series, especially following after said series' 35th.
I just wouldn't necessarily expect FP9 to be when we see a Mario character, is all. Typically some of the DLC newcomers came later rather than sooner regarding whatever they advertised, especially Min Min and even Byleth. So I wouldn't be shocked if they showed up last.
Well, also, it's probably just that MM/2 doesn't really have much of anything interesting to add that wasn't already seen in other Mario games beyond, what, the Undodog? Maybe we'll see that if there's a spirit event about dogs, I guess.