1. I don't think DK is more popular than Kirby, coming from a guy who owns more DK games than Kirby games.
2. What new Kirby stuff have we gotten besides a stage and an assist?
1. Sales wise, Donkey Kong is more popular than Kirby. However, if you exclude any spin-off titles from Kirby, and only focus on the actual platforming Donkey Kong games, Kirby has a chance of being more popular. I'm not sure, though. Donkey Kong is a lot more known than Kirby, and I don't think anybody could deny that.
2. While we may not get that much new content, here's a little comparison.
Kirby:
Characters -
Kirby, King DeDeDe, and Meta Knight
Stages -
Halberd (returning, Wii U) and Dream Land (new, 3DS)
Items -
Dragoon, Maxim Tomato, Star Rod, Superspicy Curry, Warp Star
Assist Trophies -
Knuckle Joe (returning) and Nightmare (new)
Smash Run Enemies -
Bonkers, Bronto Burt, Gordo, Parasol Waddle Dee, Shotzo, Plasma Wisp, Tac, Waddle Dee, Waddle Doo
---
Donkey Kong:
Characters -
Donkey Kong and Diddy Kong
Stages -
Jungle Japes (returning, 3DS)
Items -
Hammer, Spring, Banana Peel
Assist Trophies -
N/A
Smash Run Enemies -
Blue Critter, Green Critter, Tiki Buzz (Critters are basically the same enemy, so I'd count this as only two enemies)
---
As you can see, Donkey Kong, despite being one of Nintendo's most popular franchises, and probably one of Nintendo's most known franchises, is severely lacking in content. Whereas, Kirby, while being a popular franchise, isn't one of Nintendo's most popular (although iconic), and isn't really known too well by the average consumer, yet has tons of content.