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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.