I've always said it's better to look ugly in a picture on purpose than without trying. When it comes to Cara, though, she's a model, she literally makes a living off of people taking serious pictures of her. Why not make a dumb face when given the chance?
I think if someone knows what they actually look like then it's a good thing. It makes them not look like plastic people that are just full of themselves but that they actually look like an easy going down to earth person.
I feel like most people who always have to make faces like that are insecure or they just want all the attention to be on them. However, there is nothing wrong with genuine silliness. Everyone wants funny friends.