She looked awfully happy to me in both the end of Suited for Success and Best Night Ever. If you're talking about when she first saw the dress in Suited for Success, none of the girls were happy about their dresses, and that was the point of the episode. They all learned to respect their friend's hard work and the dresses that Rarity made specially for them.Rainbow Dash has never really expressed distaste towards dresses (AJ has, but not Dash). I mean, she's worn quite a few of them throughout the show and seemed pretty content. As long as the dress suits her style and makes her look cool and awesome, then I don't think she has a problem.