Boo feels like the odd one out in the Mario set. He's the only enemy there. Would've made sense to include him in a future wave with Goomba, Koopa Troopa, Shy Guy etc.
Boo is one of the most popular Mario enemies, though, especially when it comes to being playable (I think Dry Bones and Shy Guy are the only ones more popular in that regard). Plus, they have a hook with Boo, making it glow in the dark. I don't know what they could have done with a Goomba that would've made it special outside of being a Goomba.