All [Link] does is cut people with a sword. That's it. And you're complaining about Diddy's... why?
Are you not remembering the part of Link's Final Smash where
his enemy is trapped in the triforce? It's not like it's just Link flailing at someone—it's Link flailing at someone
with the help of the triforce to immobolize his opponent. There's a certain level "superness" to that. Diddy's, in my opinion, as wells as a few others, does not have that. Even if it is cannon with a game, it just isn't something that seems like it should require a Smash Ball.
It's imaginative, cool-looking, and unique. What else do you want?
That's just a lie.
It's
not imaginative, because it is almost literally just his standard special and up special combine. Admittedly, yes, these peanuts seems to be altered into explosives (I heard a while ago that it's actually possible to do that with peanuts. Don't know how much truth there is to that), and that's a step in the right direction, but it's still essentially the same thing. Unlike, say, Samu's final smash, where the blast basically takes up the whole screen and transforms Samus, the peanuts seem to essentially do exactly the same thing they normally would and cover the same amount of space. They probably just do a bit more damage and a bit more knockback, if anything.
Whether it looks cool or not is subjective. Though I do agree with you.
And it's not unique. It's quite the opposite actually. No one's played the game yet so it's impossible to tell for sure, but Diddy's final smash looks AWFULLY similar to Yoshi's. That automatically robs it of any uniqueness it may have had.