You can design a mobile app on your own if you have some technical skills and are willing to advance them as you go. Alternatively, you can use zero-code solutions. They let you develop simple native and cross-platform applications by customizing existing templates. Additionally, you can use generative AI tools to help you design the mobile app UX and prepare it for launch.
At the same time, you may need professional help to design a really smooth and user-friendly UX. Relying on your own skills to design an app may reduce the initial development investment, but it can also be detrimental in the long run, especially during a fundraising stage.