Tuned mass dampers (TMDs) are widely used to mitigate seismic vibrations. However, studies have shown that TMD effectiveness diminishes in nonlinear systems and varies under different ground motions. This study investigates TMD performance in elastic and inelastic single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) structures. Numerical models were constructed in OpenSees and subjected to 20 ground motion records. A parametric analysis examined the influence of TMD stiffness on displacement and inter-story drift using nonlinear time history analysis. The TMD stiffness ratio was systematically varied to identify an optimal value that maximizes response mitigation in nonlinear systems. Results indicate that traditional TMDs, tuned to the structure's fundamental frequency, often underperform in nonlinear systems. In some cases, they may amplify responses at higher ductility levels. For SDOF systems, reducing TMD stiffness below traditional values noticeably improves performance. Optimal TMD stiffness was determined by minimizing displacement ratios. For MDOF structures, reducing TMD stiffness below the traditional value decreases the response ratio in the upper stories of elastic structures and in all stories of inelastic structures. Notably, optimal TMDs selected by minimum total story displacement ratios differed from those selected by minimum inter-story drift ratios. This improved stiffness-based tuning strategy was subsequently applied to a Tuned Structural Mass Damper (TSMD) for nonlinear systems. The TSMD utilizes a portion of existing roof mass, thereby eliminating the need for additional weight. For SDOF systems, the TSMD demonstrated considerably better performance. In MDOF systems, TSMDs reduced inter-story drifts in inelastic structures and mitigated drift amplification in upper floors.