Currently, there is no built-in option to switch survey buttons to smiley faces instead of numbers or text.
However, in CSAT and CES survey templates you can edit the text written in each button and replace it with actual emojis (as emojis are supported in text fields).
For instance, your buttons might have the following emojis:
😠 🙁 😐 🙂 😀
And here's an example of how this would look like in the survey template editor:
And the emojis will be displayed in the style supported by the email client or browser the customer will use to access the survey.
Here you can see a template with emojis and white buttons background (opened in Gmail in Chrome):
And here's an example with the same template but with colored buttons: