Focus your answer on the behavioral traits of a designer versus skillset. Good design isn't about being an expert in Adobe illustrator, it's about creating a design that fulfills the goal of a project.

This is another question that speaks to the importance of researching the agency/company. Understanding the kind of work they create will give you insight into what they consider "good design" which you can use to tailor your answer on what makes a good designer. Many interviewees will want to talk about how creative they are because they think that is what makes a good designer. However, it isn't about being incredibly creative, it is about creating design deliverables that fulfill the goals outlined by a client or contained in a creative brief.

A good designer is someone who asks questions and focuses on the goals of the design. Design should be purposeful and thoughtful. A strong skill set and creative ideas are important, but it is ultimately about creating design that takes into account everything laid out in the creative brief.