I don't mind the dress, though it's not really to my taste.
That said, I think that an argument can be made that it wasn't maybe really appropriate, for reasons that don't probably make sense unless you're Finnish. Independence Day is a celebration, but not really in the sense I imagine it's in the US. It's more of a remembrance as we came very close to losing our independence during WWII and Independence Day is mostly about that. The only parades that happen are either military or students with candles who march to graves of fallen soldiers (Finland retrieved most of the bodies so all cemeteries have an area of "hero graves"), there's always a famous war movie on tv and the biggest event is watching the party at president's palace on tv (I think that about a quarter of population watches it live).
It is mostly a frivolous party but even it is partly about the war veterans (I think that we should also find some other forms of celebration but for many people Independence Day is only about the veterans and their sacrifices). So before the party the tabloids run stories about who's invited (certain people are guaranteed invitations, like members of parliament, both domestic and EU, diplomats, representatives of churches, police force, military and so on. There will also be some people the president has met on her travels, artists, athletes who have done well, business people, war veterans, all in all 1500 guests) and what they will be wearing. Some people will say that the party is a bad idea and takes away from what Independence Day should be about, it's a very serious day after all. Then there's the party. It's incredibly boring tv, there's couple of hours of people shaking hands with the president, hoping that someone will wear something crazy and then there are interviews about what Independence means to you (mostly they are grateful for the sacrifices of the war veterans). It's pretty much the same every year (except next year we'll have a new president but the party will still be there). Then the next day the tabloids and newspapers have pictures of the dresses, polls about who was the prettiest and stories about who had the craziest outfit and then some people will be upset because people weren't respecting the serious occasion and that it shouldn't be about them standing out but about honoring the work of the people who had built the country. There will always be people who wear crazy clothes but it isn't really a red carpet event (despite the red carpet) in the sense of Oscars or something like that so I guess that if it can argued that if you can't wear clothes that are too crazy at a wedding then you shouldn't at that party. Personally I don't really mind and have only watched it on tv because of the outfits. There were some things this year that did break etiquette, like these ones
(the first ones are about having your hair down which is apparently awful. Then there's a woman in a suit that isn't formal enough and a member of parliament who always dresses like that).