This is going to be something more directly addressed to the new content rather than Dark Pit himself.
I've never cared who has more than who when it comes to Smash Bros., I love almost every franchise in it and always approach this game with the mentality of a Nintendo fan instead of a fan from a particular series, any new character no matter from what franchise is a welcome addition for me. I admit that I love Kid Icarus (Uprising, don't care that much for the other two entries) as much as DK (DK Country is STILL one of my favorite game trilogies next to Metroid Prime).
Thereby I have to say, it isn't really that bad that we have a bunch of content from that series. I don't think either that content in Smash should be proportional to the amount of games and lifetime sales that a series have; it's something that is not written in stone and adding content that way would become extremely restrictive if we went by that.
But we should also look at things in retrospective. Kid Icarus was a very neglected franchise for over 20 years, while it's sister series Metroid and others that would later become Nintendo mainstays prospered. It wasn't until 2012 that the series came back from the dead in the form of Uprising; the game was a very deep experience that pretty much compensated for it's long absence on Nintendo consoles. Since Sakurai had no intentions of making a sequel later on, he expanded it's world exponentially and made sure that the game he made had an everlasting universe that would be memorable enough. Thus, the future of the series is uncertain but at least Uprising will remain as a shining beacon for the entire series.
On the other hand, Nintendo fans can always expect a new Mario, Zelda, Metroid, DK, Pokemon, Kirby, etc. to make an appearance on the current and next console generation. Those series are very well-know and beloved right now that there is little danger for them to fall into obscurity. So basically my reasoning is, why not throw a bone to the little guys once in a while? Kid Icarus had a LOT to add from it's universe into the Smash series from just one game that it would have been a shame if that went to waste. And can we really blame Sakurai for wanting a back-from-the-dead franchise to be known to everyone instead of letting fall into obscurity again? After all, after this game is done, Kid Icarus's history will be remembered only from Smash and Uprising while the other series will have a whole legacy, past and future, to rely upon.
Don't misquote me on the content part by the way. I admit that having a couple of assist trophies from DK like Rambi would have been awesome, but personally is something I'm not gonna fret about. After all, it's not like the existing content took the place of other potential content.