How good design could hurt your app
If you have created a well designed prototype or mockup, its much more difficult for potential users to be objective and provide honest feedback. They will get distracted by the beautiful design (or lack of it), and subconsciously base most of their feedback on how it looks/feels. What you would really want at this stage, is for them to understand and tell you if the core of your product is worthy and solves a real, genuine problem for them.
Design is important, but the timing to focus on it is also critical.