the servers of marios strikers charged football are completly overloaded - i can't even get online, and if it works, i drop 10 sec after the first game started.
now imagine how overloaded the servers for ssbb would be if there was an online-mode in which you can play against the rest of the world (or probably the rest of your continent). it just wouldn't be playable, even for those who play for fun and not for skill.
i personally don't think that there will be such an online mode. you'll probably be able to download the best times for break-the-targets or upload your own times, but that's it.
but hey, everyone says that ssbm is perfect, and it didn't have an onlinemode (ok, noone expected that - it's just an example to compare the games). so why can't ssbb also be great without it?