Nintendo's online has gotten better since Brawl. Mario Kart Wii is much better than Brawl ever was.
I think Brawl was a game that was too fast, with too much going on to work well online...but they tried anyway, and it was foolish.
It also did not help that Nintendo had a horrible matching system, underwhelming servers, many other problems.
I think Nintendo has learned many lessons from Wii online, but are also somewhat stuck with the online structure. Perhaps with the 3DS they can build a better experience through better infrastructure and tech.