Well, not every Mario game needs every standard enemy. I mean Mario Sunshine didn't even have Goombas, among other standard Mario enemies. But that doesn't make it worse of a game, or even just less of a Mario game.
On the contrary, I think it's good, because it makes room for new creations, which could potentially be new classic enemies, new fan-favourites or simply lead to new and interesting game mechanics.
Like the Pokios! Aren't they pwecious?
Or this thing:
Or this guy:
Would he have been better, if he had just been another Thwomp variation? No, he would've been boring.
So I'm glad they made an entirely new design for him.
There are so many Mario games. And so many of 'em have Boos in them. So I'm not gonna be disappointed just because they weren't in one game.
Besides, we've already seen what it's like to play as a Boo.
And besides besides, if an enemy hasn't been used in this game, maybe it'll be used in Odyssey 2!