Is it wrong to wear this to prom?

I'm a generally laid back girl, I don't spend a lot of time getting ready in the morning, I don't wear makeup and I wear my boyfriends shirt and some leggings almost every morning. I'm a pretty standard laid back look. But, my boyfriend is a junior and I'm a freshman, and he asked me to prom. I don't want to dress very fancy, I plan to wear my black Doc Martin combat style boots as my prom shoes. But to go with my laid back look, I want to have a more casual dress. (The theme is breakfast at Tiffanys, so I would have gotten a little black dress but there weren't any that fit my style). Does this dress look too causal for my first prom?  (It's not my size, so the initial dress would be more snug on my body)