People are looking into it, but I don't think there's all that much promise. Fine structure of shielding doesn't really matter to an energetic particle that's blowing through meters of it.

Gamma rays are stopped by electron density. Electron density requires high mass density heavy (high Z) nuclides.

Neutrons are stopped by light nuclei via conservation of momentum, and by neutron absorbing nuclei like boron.

If metamaterials can be made with higher density electrons in a way that's cheaper than lead and high hydrogen density that's cheaper than concrete, paraffin wax, or water, then I guess it could be interesting.

I used to work on novel shielding designs. There's some pretty interesting stuff out there but in the vast majority of situations it is just cheaper and easier to pour more concrete.

To add to acidburn's comment, material choice is also highly dependent on the energy levels and particle types. At high energies you basically just maximize mass. At lower energies (more typical for reactors) you can start taking advantage of atomic cross-sections and electromagnetism. That's why they like borated concrete, high neutron cross-section, but only really effective for "thermal neutrons".