That's not really new since people have been saying that for years. Its died down a lot since Brawl.
Really, you can boil down what matters into three factors
- Is the Character popular?
- Is the character unique in some way
- Is the character relevant (as in they have been seen in the last few years)
How much they matter differences, but for the most part you can look at these and they apply to every character added. The last one is important since, besides the guest and classic characters, everyone has fit that criteria.
Brawl
- Meta-Knight - Appeared in a lot of the Kirby games since Nightmare to Dreamland (2002)
- Zero Suit Samus - From Metroid Zero Mission (2004 or 2005)
- Wario - Been around, but the design is based of Wario Ware which came out in 2003
- Ike - From Path of Radience (2005)
- Pokemon Trainer - Pokemon Fire Red and Leaf Green (2004)
- Diddy Kong - Been around. Was appearing in Mario spin offs as well as King of Swing and Donkey Konga.
- Lucas - Mother 3 came out in 2006
- Olimar - Pikmin started on the Gamecube in 2001 and had a sequel in 2004
- Lucario - From the newest Pokemon game Diamond and Pearl
- Toon Link - Really just Young Link but he was based off the Wind Waker (2003)
- Wolf - Was in Starfox Assult and Starfox Command (2005)
Smash 3DS/Wii U
- Villager - Animal Crossing was Nintendo's biggest series without a character at that time. Last one was New Leaf which was 2013
- Wii Fit Trailer - Wii Fit - 2008
- Rosalina - She appeared in a ton of the Mario games: Galaxy 1 and 2, 3D World, and a lot of the spin-off games
- Little Mac - While technically considered one of the classic characters, we got a game in 2009
- Greninja - From the newest Pokemon game X/Y (2013)
- Palutena - Appeared again in Kid Icarus Uprising (2012)
- Robin - From Awakening (2013)
- Shulk - From Xenoblade Chronicles, new series that started in 2010 (in Japan)
- Bowser Jr - He was appearing in a lot of the Mario spinoffs but hes based on New Super Mario Bros U (2012)
- Lucina - Awakening
- Dark Pit - Palutena
- Corrin - Fates (goes without saying as he was added for being new)
As you can see, a lot of the roster is from newer games. That seemed to be a higher priority for Smash 3DS/WiiU, but it still happened for Brawl. None of the characters besides the classic character didn't have an appearance between the titles before it. That could change, mind you, but you have a better shot picking from the new characters. At a bear minimum, you might be closer to 50% if you follow this method as at least you get some of them.
This was a bit off topic, so let me tie this together. What Omni is saying is don't look at dumb criteria. We have never seen "reps" being a determining factor for adding characters. Heck, Fire Emblem got 3 new characters in this game. Something like "being current" does seem to matter. You could argue the same thing about popularity. During Brawl, being "popular" was hot. But we come to find out that just because a character is popular doesn't mean they'll be added (heck, look at Ridley). Something like using "current" characters will probably work better (though it doesn't guarentee that you'll be accurate).