Honestly, Super Smash Bros kicks Mario Kart's ass when it comes to being fun to play.
I wish I agreed with you, but that just hasn't been my experience with Smash Bros this year. Then again, I think you have a group in-person to play with? So I'd probably feel different about it then.
I need to add more friends on my Wii U so I can at least have mic chat between matches, 10-minute matches, crazy item set-ups, all the fun stuff you can't do on general online. Maybe then I'll start having a good time.
On the flipside, I can enjoy Mario Kart alone, with someone with you, online with friends, online with randoms. Fun not just one way, but lots of ways.