I'm thinking about telling my employee that she should be more cheerful.
Is that what's on your mind? Is she not cheerful enough?
I'd like her to be more energetic when she speaks with us
If that's part of her job, then I totally understand. Giving feedback to employees so they can do their job better is a good idea. It'll help them, and help your business!
No we just want her to smile for us and speak in a happy tone. I don't see how that would be a big deal.
I agree, I don't think it's that big of a deal. It's normal to have expectations of your employees, especially employees that deal with customers. It's good that you're giving her feedback so that she can improve and your business can as well.