A multi-tier HCN comprising MBS and PBS is employed to enhance network capacity and meet the high data rate demands of 6G wireless systems. To improve spectral efficiency and support a dense user population, NOMA is adopted as a key access technique. This study presents a comprehensive performance analysis of HCN-NOMA, taking into account practical impairments such as i-SIC and i-CSI. Leveraging stochastic geometry, the analysis evaluates and compares the performance of OMA and NOMA in multi-tier HCNs under both ideal and impaired SIC scenarios. Key performance metrics include outage probability, asymptotic outage probability, system throughput, and energy efficiency. The results demonstrate that, even under realistic conditions involving i-SIC and i-CSI, NOMA consistently outperforms OMA, achieving performance gains of up to 57%. These findings underscore the robustness and potential of NOMA in practical heterogeneous network deployments.