This study employs density functional theory to explore CsSrI3 and its Si- and Ni-doped variants for metal–insulator–metal (MIM) capacitor applications. Key polarizing indicators—including dielectric constant, electron density, energy density, Born effective charges (BEC), and loss tangent—exhibit characteristics suitable for high-performance MIM dielectrics. Negative formation energies confirm thermodynamic stability across all systems. Doping effectively narrows the pristine CsSrI3 band gap from 3.9 eV to 2.1 eV (Si) and 1.9 eV (Ni), enhancing electronic conductivity. BEC analysis shows increased Zzz-direction polarization for Si doping, while Ni introduces a stronger positive effective charge (∼+2.5 e), attributed to Ni d-orbital interaction with halide p-states. Charge-efficiency analysis reveals Ni contributes 9.7%, surpassing Si, which contributed 5.7%. Charge-density mapping demonstrates dopant-induced delocalization, with Ni producing pronounced anisotropic accumulation. Ni-doped CsSrI3 also yields the highest electrochemical coupling (3.72 × 10−11 mV) and improved capacitance (360 μF cm−2), identifying both doped systems as promising MIM capacitor materials.