The outfits are representative of the age of music they're playing. Sunset Shimmer is the only one with a 'new look' as she has the same outfit in both videos, and doesn't change at all for the different musical periods for Friendship Through the Ages.
I just realized what you meant by this, and what the video is going for. Previously, I didn't even realize that this video had a theme.
-Things may come and things may go
-Some go fast and some go slow
-Few things last, that's all I know
-But friendship carries on, through the ages
They're showing a lasting friendship (with Sunset Shimmer) through the use of dated musical fads. But then, I don't think they got it right.
It starts off with Sunset sitting on Twilight's grand piano. "Girl on piano" is a thing, although it's usually a sexy/romantic thing (hence the reaction of "Woo hoo! Twilight/Sunset shipping!"). And in the most common traditional version, I'm pretty sure it's supposed to be a handsome guy playing to a beautiful girl, while the beautiful piano-sitting girl is the one singing to the handsome guy (or maybe the audience). Basically, Twilight took on both roles, to direct her
love friendship towards a passive Sunset. It's odd. And then, Twilight's costume is pretty obviously inspired by the past (which past? It seems pretty hard to put a date on it), but it's not authentic. The piano key designs on the skirt look more like a Rarity creation than anything dated. So it's like, Twilight is making subtle complimentary nods to the past, but not effectively referencing the past. And it's not like the grand piano is dated anyways. The very next thing this franchise did was have Rarity proclaim that the grand piano is and always has been the height of piano fashion for those who can afford it, and then have Lyra and Bonbon getting intimate over one. Basically, this experimental video suggested that a grand piano was dated, but then the rest of the team said "No it's not, BTW, you've just made me realize that it's super awesome for shipping."
Next we have Fluttershy in the mountains and... I'm guessing Sound of Music? But the song doesn't fit, or really change much at all, and Fluttershy isn't evoking an image of Julie Andrews. And I'm not sure anyone today would dare to call Sound of Music a fad. Maybe Fluttershy just likes the peaceful outdoors? Basically, I don't get what the video is going for here.
For Rarity, she's dressed like she's in a marching band, or maybe inspired by Michael Jackson, but she's climbing stairs of giant flowers. I don't get it. And the music is again giving no hints.
Rainbow Dash is obviously rock music, and she's smashing guitars and Sunset is crowdsurfing, and these things were sort of new in the 80's, but they haven't exactly gone away. Especially not with how well Rainbow Dash falls into the role. I should note that the music is significantly tweaked for this scene, giving it another level of obviousness.
Applejack is country music, but country music isn't even close to representing a time period, especially not one that ended. And Applejack embodies the role too well. She could've just walked into the frame and said "Hey y'all. Are we filming a movie or something?" and you've got your country music video right there. The music has been sufficiently adjusted to say that yes, this is country music.
Then Pinkie Pie goes full 80's. IMO, this is the only part of the video that works properly, and Pinkie Pie nails it. But if Pinkie Pie is 80's, what is Rainbow Dash?
Basically, I think this group video is the opposite of Sunset Shimmer's solo video, which I think was very focused and deliberate and worked on every level. Unless there's a lot of stuff that I'm still not "getting" about this group video.
