80s Oinker Sisters, again, were pretty much a reflection of 80s all-female punk or rock groups of the day (Bananarama, The Go-Gos, The Bangles, what have you), so that made the reference pretty relevant at the time. I always found their transformation into pig versions of The Supremes in the late 90s and early 2000s an odd direction to take the group into, considering there were a number of really prelevant genres and sub-genres of music in the 90s they could have very well fit into.
But then again, there's also the matter of The Squirrelles, who were around in the 90s, and basically were nods to 60s female do-wop groups like The Shirelles and The Crystals, so . . . I don't know, I guess the production/writer/music staff of the show at the time felt compelled to pay tribute somehow to 60s acts, who knows?