Since Melee is turning 25 this year (and since AC technically made its debut in that entry with a couple of trophies), I've decided to do this discussion with most of the relevant franchises featured in Smash.
For context, here's the full list:
Title Theme--Animal Crossing [GCN] (Ultimate arrangement, by ACE)
Radio Exercises (Brawl lost track; sound file does not play; believed to be the original)
Go Go K. K. Rider! (Brawl arrangement of both versions, by Keigo Ozaki)
Title Theme--Animal Crossing Wild World (Ultimate arrangement of the title, 3 PM, and 2 AM from Wild World, by Maki Kirioka)
Title Theme--Animal Crossing Wild World (Brawl arrangement of the title, Museum Entrance, 3 PM, Loan Paid Off, 6 AM, Nook's Cranny, and Town Hall from Wild World, by Kazumi Totaka)
2:00 AM--Animal Crossing Wild World (Brawl arrangement, by Tomoko Sasaki; technically debuted in a soundtrack CD two days before)
Town Hall and Tom Nook's Store--Animal Crossing Wild World (Brawl arrangement of both songs, by Kentaro Ishizaka)
The Roost--Animal Crossing Wild World (Brawl arrangement, by Kazumi Totaka)
Plaza/Title Theme--Animal Crossing City Folk/Animal Crossing Wild World (Sm4sh arrangement of both songs and 2 AM, by Kumi Tanioka)
Outdoors At 7:00 PM (Sunny)/Main Street--Animal Crossing New Leaf (Sm4sh arrangement of Arrived At The Village, 7 PM, Opening Theme, and Main Street 3 from New Leaf, by Manaka Kataoka)
Kapp'n's Song (Sm4sh arrangement of both Kapp'n songs, by Shohei Tsuchiya)
Tour--Animal Crossing New Leaf (Sm4sh arrangement of both Tour themes from New Leaf, by Hajime Wakai)
Tortimer Island Medley (Sm4sh arrangement of Tortimer Island Lobby, Tortimer Island Day, and Tortimer Island Tour, by Masafumi Takada)
Bubblegum K. K. (Sm4sh arrangement of all versions of the song, plus Opening Theme from New Leaf, by Atsuko Asahi)
Title Theme--Animal Crossing: Happy Home Designer (Ultimate arrangement, by Kazumi Totaka)
House Preview (original; Phase 1 only)
In addition, in every game the Smashville and Town and City stages have appeared in, K. K. Slider will appear on those stages at the same time that he appears in all Animal Crossing games (Saturdays, from 8 PM to 12 AM), and he will play the original version of one of six songs:
K. K. Cruisin'
K. K. Western
K. K. Gumbo
Rockin' K. K.
DJ K. K.
K. K. Condor
Yeah, this is about as good as they could do for the most part. Outside of the K. K. stuff, this is basically every relevant song that could've possibly been used for Smash, and even then, it's almost all rearrangments. I have only two things to say: K. K. Disco should've been here, and House Preview should've been the full version. Other than that, it's about as good as it'll get for this franchise.
Let me know what you think, though.