Yeah, that would make a lot of sense. And it would explain some things like how she covered up her breasts, etc. My memory is a bit bland, but have we seen any such hybrid forms in the past where it looked half human, half animal? I think currently Black Maria might be the only one? I'm still a little iffy on it, but it can work with how Black Maria has her illusion mist.
Or it might be what Deicide was stating that the top portion is an illusion, and maybe that's why it was covered up that way. Maybe it's a hint/clue on the top half being an illusion, and the spider being the real Black Maria? I guess we have to see next chapter.
I don't know about it being an illusion specifically. She's up there swinging Wanyudo around, damaging the environment and lighting fires. Brook would have noticed something if he tried to attack Wanyudo and found nothing there. So there's obviously something physical up on top doing the work, and its physical strength is not insignificant. A web double I could maybe see, but I would have been a lot more receptive to this theory before this chapter came out and we saw her doing so much of the fighting.
Oda's totally willing to break the Zoan rules when the animal doesn't anthropomorphise easily, or if he has a fun idea for something different. Suppose we say the "normal" hybrid form is like Kaido's, with the animal-patterned arms and clawed human hands, a cowling from the nose up and a tail depending on the creature. Then you've got Chopper, whose huge range of forms has none that fit the hybrid expectations (or a fully transformed space); the Boa sisters, whose presumed hybrid modes include full snake torsos with human arms and breasts; the differences between Pell and Marco, with the latter's hybrid mode looking head-to-toe like a humanoid bird, including avian feet replacing his hands, while the former just has wings replace his arms and a tail out the back; or Kaku, who got full-body fur, hooves on his hands, a long neck and a weirdly cube-like set of shoulders.
The choice to make Black Maria a spider centaur is not one that stands out to me as a loose end or deliberate hint at something more. I could be wrong of course, but that angle isn't really vibing with me.