TSR's Super Smash Bros Thread|SWITCH HYPE TRAIN NOW BOARDING

[Pik]

Lucina and Doc are the only ones that need extensive model work, but even then they were originally alt costumes according to that Famitsu column Sakurai wrote. It was just a matter of making those costume models as new characters. It wasn't effortless, but it was content they already had to work with. (So basically, the modeling work could be compared to the Alph costume)
As for Dark Pit, he's just a simple texture edit of normal Pit-- His face is edited a tad, but it's pretty minor. So getting their models was easy.

Lucina for one reuses a lot of Marth's animations, not sure about the others.

Balancing their movesets is where the hardest work comes in, I agree. That's the meat of making these clone characters.

So in short, the three clones were simpler, last-minute additions. I don't think their addition hurt the roster any, so I welcome their inclusion. (despite my dismay for them at first) Were they effortless? nah. But they were easier than other newcomers by a long shot.