I accidentally let slip out my opinion of our competitor store to a customer.
Nothing, unless she specifically talks to you about it. If she does, let it be an opportunity to offer your input on how to make your store improve, just like that "cuter" competitor. There's not even a need to bring it up if she doesn't even mention about it.
Nothing. If they are cuter and she asks you about it, offer some ideas or suggestions on how you can compete visually against them.
Don't lie and say you didn't say it. Don't defend yourself. Say, "I think we can do better and here's how."
I'm a manager. I want my people to compliment other places if they like them but I want them to come up with creative responses to us being beaten, looks or otherwise.
And your manager send out undercover customers??
how is the manager going to find out. do your customers normally tattle on you