Yes, I actually found a way around it. You can use a text widget instead of the enews widget and simply copy and paste the HTML code for the sign up form into the text widget. Or download Mailerlite’s plugin and copy the shortcode into a text widget. The Genesis Enews plugin isn’t compatible with Mailerlite (according to Mailerlite).
By using the complete HTML code for a sign-up for in a text widget you can then go and alter things like the font of the signup form individually.