I'll concede on Lucina because I've never played Awakening, and don't really know all that much, just going off of what other people say.
Dark Pit and Pit are extremely different though. Their personalities are basically polar opposites and their fighting styles are not that similar either, Pit is more heroic and stuff while Dark Pit doesn't really hesitate to take cheap shots (kicking Pit after finishing Pandora, abusing his power of flight, etc.)
Dark Pit and Pit really shouldn't be clones.
Yeah, their personalities are different but that's shown through taunts/victory animations. It's been stated that Dark Pit and Pit are identical in their abilities and the animations for Pit's moves don't look out of place on Dark Pit so it's believable.
It's not at all believable that Shadow would fight like Sonic, though. The former is always no-nonsense and has a brutal, Chaos power-based fighting style. The latter is a big show-off even in combat, likes playing around with his enemies, and has a fighting style that heavily relies on speed.
Sonic's animations would look awkward/dumb on Shadow because, for the most part, they're very silly and playful. When they create clones, they purposefully try to find characters that are similar enough to one on the current roster so that it at least makes
sense for them to fight like each other.