A Mixed Reality (MR) system encompasses various aspects, such as visualization and spatial registration of user interface elements, user interactions and interaction feedback. Image-schematic metaphors (ISMs) are universal knowledge structures shared by a wide range of users. They hold a theoretical promise of facilitating greater ease of learning and use for interactive systems without costly adaptations. This paper investigates whether image-schematic metaphors (ISMs) can improve user learning, by comparing an existing MR instruction authoring system with or without ISM enhancements. In a user study with 32 participants, we found that the ISM-enhanced system significantly improved task performance, learnability and mental efficiency compared to the baseline. Participants also rated the ISM-enhanced system significantly higher in terms of perspicuity, efficiency, and novelty. These results empirically demonstrate multiple benefits of ISMs when integrated into the design of this MR system and encourage further studies to explore the wider applicability of ISMs in user interface design.