Yeah, I kind of agree with this, Donkey and Diddy's movesets are pretty bad.
I never play as them because they don't feel like the characters I liked as a child, just some... animals.
Samus and Kirby have similar problems, very boring movesets considering the incredible amount of potential they have, but they are "fine".
Like why can't Samus shoot her projectiles like Mega Man (multiple at the same time), why can't she charge in the air?, why can't she use Screw Attack more than one time like Witch Twist?, Why doesn't she have ice in her moveset?.
Kirby is just boring, 90% of the moveset are kicks that have the excuse of being from the Fighter ability, but yeah, boring... and bad.
Pikachu also feels kinda weird for me, it tries too hard to be cute, and I like Pikachu thanks to the anime, where he is super strong and likes to battle. Also no Iron Tail, and Volt Tackle being a weird floating ball.
Also, Jigglypuff, yeah, Rest, what is that,
Sing, her more iconic move, being totally useless, etc.
All the very unique and "kinda canon" movesets from the Smash 4 newcomers made me realize how bad some of the old ones are.