Robin's smile fades slightly when Sans asks that - more out of confusion than any sort of unhappiness. She doesn't really understand the question.

"Well, yeah. They're my body parts. Why wouldn't I use them when I wanted to?" She understands that a lot of people think they're creepy, of course, but they're as much an extension of her body as anything that's permanently attached. She decided a long time ago that she wouldn't avoid using them just because some people didn't like them. It wouldn't make sense for Sans to avoid using his arms just because people didn't like seeing his creepy skeletal fingers, would it?

"I can use them for anything. Sometimes it's just fun to scare people with them, though."