Dr. Mario is widely considered to be worse then Mario...
Dark Pit had the least amount of effort put into him, undeniably, but he's probably the most useful with his Electroshock in certain matchups. (Mac, Ike, LUMA.)
Dr. Mario has a ton of unique attributes... But they really don't amount to anything. The real killer is his slow speed ruining his comboing and pressuring ability. Like I said, he doesn't really offer anything over Mario, not even unique matchups advantages like DP does.
Lucina is supposed to be an easier to use Marth. The problem is that 1. Marth already sucks in this game and 2. You'll deal more damage/KB overall with Marth even if you're horrible with him since the "tipper" is more then half the end of the blade now. I mean, you KNOW it's bad when her fans would of preferred her to be a costume instead of a separate slot.
While Dr. Mario is worse than Mario (no questions there), his properties does allow him a unique playstyle from his parent: His pills, while being worse for approaching opponents, is good for forcing people to approach you instead, the tornado (like Luigi Cyclone) can outprioritize certain things like Sonic's Spin Dash and is pretty mobile, he has unquestionably more general power on his moves (%, knockback) than his parent, and his Up B is good for attacking, and he can do simple combos (though Mario has better comboability). His flaws in comparison is his poor mobility and recovery, and Mario undeniably gimps better than him. Of all the clone pairs, I think Dr. Mario got the rawest deal in terms of setting himself apart from the original, and picking between the two will likely be up to how you play.
Dark Pit being mostly unchanged from Pit, while it makes him worse, mostly works in his favor: Electroshock is preferred in certain matchups, and while his arrows are worse than Pit's, I don't see how they are bad as a projectile. Of all the clone pairs, Dark Pit will likely be the closest to the original, tier-wise, and he gives the best deal in terms of who you prefer appearance-wise in a clone pair since he's so similiar to Pit while still having a couple of his own quirks.
Lucina probably got the worst deal out of all the clone characters: Her most defining trait works against her in terms of getting advantages over her counterpart, Marth's tipped and untipped attack hitstun properties make him not as punishable if the opponent shields his attacks unlike Lucina who has to be more careful, and she doesn't really have a niché outside of being easier to use (at least in the beginning).
