I'm surprised at the "too casual" comments. I think it is a lovely dress. I think the right accessories would do a lot to smarten it up and make it look even less bride/bridesmaid-like. I'm not a fan of DM's with dresses though.
I’d be upset if my MIL wore this to my wedding.
To be fair @PacifyLulu ^^ this is why I mentioned self esteem issues. I didn't mean to offend, but that I how I read this comment.
One of the guests at my wedding wore a pair of blue and white striped jeans. They looked like pyjama bottoms. I wasn't offended, but the thought that crossed my mind was "I've dressed up for the occasion, why couldn't you?"