Kirby - Although Kirby goes on adventures would youu really say he's a fighter or has fighter skills?
Yes, Kirby fights in his games. Plus? There is literally a series called Kirby Fighters for both 3DS and Switch. (Kirby Fighters 2)
Peach - If peach could fight she wouldn't get taken by Bowser in every game
Peach doesn't get taken by Bowser in every game, and it's theorized she does this on purpose. Besides, she has fought in previous games like Super Mario Bros. 2, Super Mario RPG, Super Princess Peach, and Super Mario 3D World.
Daisy - She's no fighter
Have you played Super Mario Strikers? She's no push-over. That said, you're technically correct (the best kind of correct!) since she has never thrown fists before.
Mr. Game & Watch - Doesn't this guy just catch things?
He's done a variety of different things, including Game & Watch: Boxing.
Olimar - I haven't played many pickmen games but doesn't this guy just walk around collecting pickmen and not fighting?
Yes, Olimar has the Pikmin do the fighting both in the Pikmin games, *and* in Smash. Everything lines up.
Villager - AC has no fighting and your villager really can't fight
Wii Fit Trainer - S/he fights to keep the calories off and that's bout it
Rosalina - Not really a fighter but a giant space woman
Palutena - I really have no idea what she does but don't think of her as someone with fighting skills.
Duck Hunt - This guy just picks up dead ducks
Or laughs at you. But keep in mind, Duck Hunt is actually a THREE character team - the dog, the duck...and YOU using the Zapper to shoot the opponents on-screen. Duck Hunt isn't the one shooting when in Smash. It's actually the player using the Zapper. And since the player has always been the one *attacking* ....I dunno this one is a gray area.
Isabelle - She's even less likely to fight than the villager. How can you get lower that 0%?
Piranha Plant - This thing just pops out of pipes
This one was admittedly a stretch, but when you consider everything this species has done...including spitting up spike balls, fire balls, etc. To say it's not a fighter isn't quite correct, either.
Steve - On the fence with this one, you do "fight" in minecraft but it's pretty basic.
You said it yourself, you fight in Minecraft.
My points still stand.