Control of type 2 diabetes (T2DM) is based on patients’ diet, physical exercise, and medication adherence. Mobile applications can help patients with T2DM better manage their condition. Before one mobile app is launched on the market, evaluating its usability and alignment with patients’ needs is essential. This study aims to determine the usability and satisfaction of a mobile application and assess the users’ opinions in two stages of its development process. We performed a pilot usability study, and after applying the identified improvements, we carried out a one-week usability study some months later. In the pilot study, a convenience sample of 6 participants was recruited, and the one-week study was performed with 22 participants. Both studies used questionnaires to assess usability and satisfaction, and the users’ opinions were gathered. The usability score improved from 68.8 (IQR = 23.1) to 80.0/100.0 (IQR = 13.8), indicating good usability in the final survey. The satisfaction score also improved from 24.5 (IQR = 3.3) to 26.0/30.0 (IQR = 3.0), suggesting excellent satisfaction in the more recent test. According to the main difficulties experienced by users in the pilot usability test, several changes were made to improve understanding of the following app version. The “add meal” feature was the most commented on because of the need to enhance the food database further and the machine learning algorithms to detect food through photography. This study improved the usability and satisfaction of a diabetes self-management application and provided insights into what these patients value.