In fact, mockups are not only the final image, but a tool that helps you check how the design will look in real conditions. You don’t have to do everything perfectly from the start. At the first stages, you can use rough mockups to understand the general concept, and then gradually refine them. Also, mockups don’t always have to be super accurate at the start, their purpose is to help you understand what the final product will look like. If you’re interested, here’s a link to an article https://sbhinter.com/mockup-mythbusting-placing-your-hat-on-mockups , which explains in detail how to work with mockups and dispel these myths.