"It's only a prototype". Sure. But what's the purpose of a prototype, and do you think bugs and basic UX design failures will help you achieve that purpose? You want feedback on the soup? If there's a fly in it, that's the feedback you'll get. No point asking "Too much salt?" 'cause they won't remember that detail. Prototypes created to get user feedback need to be reliable enough and usable enough that the stuff you want feedback about stands out in the user's experience.