I agree that in theory it would be nice but most none pony people have no idea what they have and so just stick their auction where ever. I even suspect that some people who know better do this on purpose in an effort to get a wider audience.
This for me. I completely agree I would like to see it categorised better, ideally by generation but at least having G4 split out to its own category.
However, that being said I would still generically search "My Little Pony" anyway rather than just searching individual categories. For a few reasons 1) people sometimes accidentally list items in the wrong categories [how does someone know what a G3 pony is to a G2 pony], 2) people sometimes deliberately list items in more popular categories.
So all in - I think its a great idea - but it would not actually change how I use ebay.