Smart City, born from the convergence of technological innovation, sustainable urban planning, and citizen-centric governance, harnesses technology and data to enhance residents’ quality of life, optimize urban operations, and foster long-term sustainability. Smart city branding functions as a strategic instrument that shapes a city’s image by supporting and facilitating smart city development initiatives and attracting related investments. However, for branding to be effective and authentic, it must be rooted in a data-driven understanding of a city's actual performance gaps. Without a clear diagnosis of underlying weaknesses, strategic initiatives can miss the mark, and branding efforts may lack substance. Therefore, this work proposes a linguistic prescriptive framework designed to identify performance gaps in smart cities and provide interpretable, prescriptive recommendations to address them. By converting raw performance scores into 2-tuple linguistic values, the framework improves interpretability for policymakers and non-technical stakeholders. Supported by the calculation of the relative performance gap ratio, this approach enables the systematic prioritization of underperforming subareas and the generation of tailored, explainable recommendations for strategic intervention. In this work, the smart city performance of twenty-six European Union capitals is evaluated using the 2025 Smart City Index report. The findings reveal that the most critical performance gaps among the twenty-six EU capitals are predominantly structural rather than technological, with over 80% of the top five subareas for improvement relating to issues such as civic participation, traffic congestion, corruption, air pollution, and housing affordability.