When a friend shows you work in progress, your best contribution is to imagine the point of view and preferences of the person it is being created for. “I don’t like it,” isn’t useful, because it’s not for you. “I could imagine that someone who wants x, y or z would be looking for…” is much more helpful.
You don’t have to be three years old to be a toy designer, and you don’t have to be three to give useful feedback on a new design.
On the other hand, when a friend shows you something that’s finished, the most important thing you can do is find two or three truthful and positive things to say. When someone trusts you to share their work, and cares enough about the world to bring the work forward, that’s already two things worth applauding.