I do agree that for someone who knows nothing about FE a lot of the characters in Smash look way too similar to each other. They should have picked them dofferently avoid this issue, and they could have. It just so happens, sadly, that most of the ones we currently have carry a lot of significance behind them.
Marth is the og and of course he has to be here. Roy I argue could have actually been replaced by Leif or Sigurd, a scenario that was actually really likely since Thracia 776 and Genealogy of the holy War were th elast two FE games before Binding Blade, which at the time of Melee wasn't even released, so it's interesting that Roy was picked. But even then, Roy kinda represents GBA FE which is an important era for the series. Ike is massively popular and many people's favorite lord, and his games are adored by fans, and then the three Awakening characters come from Awakening, a game that "saved the series" (although I'd argue that Robin is the only absolutely necessary one among the three). Corrin and Byleth are merely here because their games were being pushed for.
So yeah, looking at it this way... you can kinda explain why they're here, but if I had to tweak the roster... Leif or Sigurd would actually be here alongside Roy (note that Sigurd fights on a horse so that's one thing I think would make him stand out), and Ephraim would fill the Lance lord role, meanwhile Ike can stay because he is fucking Ike, and Robin is pretty much the only remaining one. So yeah, you're right, quite a few of these fighters are kinda unnecessary, but what can you do.
I am slightly tempted to keep Corrin though because Green Haired Male Corrin is an excellent punching bag, which coincidentally brings us back on topic.