I have an issue with this definition. And I hope all designers do. Wireframes outline the content and features based on analytics and research. But the UI designer needs to apply a complex understanding of visual communication (including nuances in composition, spatial relationships, contrast, emphasis, variety, and rhythm) in order to support the user’s goals and workflows. They also need to understand interaction design and animation in order to fully realize the brand, product vision, and to enhance the overall direction of the experience.

Saying designers “make the wireframes pretty” is an ignorant over-generalization of their role, and fuels the misguided idea that their role is somehow less important, less intellectual or less skilled. Good UI designers tend to influence the wireframes and rework them into a better state. Sometimes the analyst that creates the wireframes is not knowledgeable in visual design, and to simply “prettify” an improperly organized composition in a wireframe is misguided.

I hope that you can add a little more to your definition of a UI Designer, and I hope that you and others understand and respect the expertise.