Considering the amount of sales Awakening had (a million or so, meaning that is the max amount of people that could be satisfied), compared to the sheer number of people who buy and play Smash (20 million and something, IIRC), I seriously doubt most Smash fans are happy with Lucina.
I don't have any attachment to the characters from Awakening since I've only played the demo, but Robin looks like a really unique addition in terms of how he/she plays. So I'm liking him/her quite a bit. I think that after Rosalina & Luma, Robin might be the most unique addition to the roster.
However, I'm not liking Lucina, as she literally adds nothing to the game. Her inclusion on the roster is just pure pandering, and probably comes from the fact that Sakurai really loves Fire Emblem, so he wants to squeeze as much from the series as he can, into Smash. I really don't think most people would care if she was just a glorified skin swap for Marth, like Panda is to Kuma in the Tekken games.
Literally the only people who can appreciate Lucina's addition are her fans, and even then, not all of them are happy with her being a clone.
Also, am I the only one bothered by the fact that Lucina is getting a free pass on being a clone, with people saying she is "just fine", while people won't shut up about Toon Link, despite the fact Toon Link is a veteran (so his inclusion comes with little fanfare or attention from the developers), and even though he is quite a bit more differentiated from Link than Lucina is from Marth?
Granted, I wish Toon Link was more unique, but because he is a veteran, him still being a clone was no surprise. And he at least has some different moves, and his specials at least have very different properties, and has a vastly different size/hit box to Link. Lucina is barely any different.