Stucco itself is not conductive, but it's usually applied on a base of metal mesh (similar to chicken wire, but apparently actual chicken wire isn't appropriate). Some projects use fiberglass mesh as the lath, and some may use traditional wood lath, but my understanding is the majority of stucco for house construction uses metal wire mesh for the lath.
Ps in order to be an effective Faraday cage it must also be grounded and no gaps in the metal may be larger than the wavelength that needs to be blocked. Which is only a couple centimetres (or around an inch or so) for the highest ones
Stucco mesh needs to have no gaps for its structural needs, it's commonly overlapped by 3-6 inches. And, depending on the surface, the mesh may embed into the ground, offering a ground connection (if poor)...
The mesh does have many openings, but they are around the size you mention, so it should be ok-ish.
Otoh, there's like doors and windows. I believe a partial faraday cage still significantly reduces signal strength though. It'a difficult to use cell networks in my parents' stucco house, but step outside and it's fine. But stucco is also very similar to concrete, so that could be it, too.