High-quality test datasets are crucial for assessing the reliability of Deep Neural Networks (DNNs). Mutation testing evaluates test dataset quality based on their ability to uncover injected faults in DNNs as measured by mutation score (MS). At the same time, its high computational cost motivates researchers to seek alternative test adequacy criteria. We propose Latent Space Class Dispersion (LSCD), a novel metric to quantify the quality of test datasets for DNNs. It measures the degree of dispersion within a test dataset as observed in the latent space of a DNN. Our empirical study shows that LSCD reveals and quantifies deficiencies in the test dataset of three popular benchmarks pertaining to image classification tasks using DNNs. Corner cases generated using automated fuzzing were found to help enhance fault detection and improve the overall quality of the original test sets calculated by MS and LSCD. Our experiments revealed a high positive correlation (0.87) between LSCD and MS, significantly higher than the one achieved by the well-studied Distance-based Surprise Coverage (0.25). These results were obtained from 129 mutants generated through pre-training mutation operators, with statistical significance and a high validity of corner cases. These observations suggest that LSCD can serve as a cost-effective alternative to expensive mutation testing, eliminating the need to generate mutant models while offering comparably valuable insights into test dataset quality for DNNs.