Yeah it would definitely give collectors the ability to only view ponies from the generation or generations they are interested in though. Ebay is super annoying. Why they use pre 1990s and post 1990s cut off is beyond me. Clearly they don't know anything about MLP.
But to me it just feels wrong for a character name to be G4 Applejack when the pony is already classified as a G4 pony.
They would still be different ponies just:
Generation: 1
Character: Applejack
Generation: 4
Character: Applejack
They could be listed separately in a character search but also together so there is the potential for an "Applejack through the years" page.