I'd say the characters are pretty consistent. It just so happens that certain characters are inherently more unique and/or compelling than others. I don't think it's a matter of laziness. It's not like any of the new additions recycle animations or anything like that -- they're all built from scratch. And the new characters are just as referential as the old ones (Toon Zelda's fear of rats, Wrecker boulder, et cetera).
I guarantee if characters like Darunia or Ruto were Legends DLC you'd be giving us the same "boring and uninspired" spiel. Skull Kid and Medli are just as interesting as them.
I find the new characters bland and lacking in engaging systems. I end playing with the more uncanny characters and weapons from the base game. The Great Fairy, the Power Gauntlet, the Book of Spells and the cubes, the Deku branch, the Time Gate, the Arrows of Light, the Wind Waker, Ganondorf and his powerful but slow attacks, Midna and her grapples, Sheik and the elemental songs, etc. And from the Twilight Pack, we had the Spinner, Young Link and they even made Tingle unique with the instant burst.
And then we have the Legends characters.
Linkle and Trident Ganondorf feel great and interesting. Trident Ganondorf can throw his weapon around and teleport in interesting ways.
Skull Kid is portrayed as plain wizard shooting black orbs and laser beams (?), instead of a tactical fighter that rely in AoE booby traps that fit his mischievous persona.
I don't know what to say about Tetra.
The King of Hyrule is somewhat fun in fast mode, but with an awful sound design that kills the character.
Marin just swings a bell around with no particular nuance. So much could have been done with her and the instruments, like Sheik did.
The Sand Rod is the Book of Spells with a "press Y to unleash train".
Toon Zelda doesn't have any differenciating mechanic at all.
I have yet to play Medli and the Pegasus Boots in detail, but I'm already soured towards the DLC characters. Fortunately, the extra maps are a nice addition.
Ruto was fantastic, her moveset based on water dances was very creative and had a simple but effective implementation of the charge bar. Darunia's attacks felt powerful and intimidating. Even if these were not the best characters around, they felt alright because they belonged to the base game. The problem is when you have to pay for extra characters and these don't deliver. When you sell DLC characters, all have to be stellar. These new characters seem to be developed by the new guy while the core team moved onto something else.