I was never able to play SSBB online except when against my own friends. Anything else was unplayable. However, I don't believe Nintendo made a big mistake. People overestimate the popularity of fighting games. According to VGChartz, SSBB is the most successful fighting game of this generation with 10.12M copies far more than any competitors.
- SSBB 10.12M
- SF IV 3.3M
- Tekken 6 2.77M
- Soulcalibur IV 2.62M
- MK vs DCU 2.42M
It seems that the expected sales for a good fighting game is in between 2.5 and 3.0 million copies. It seems that the Smash series is in a class itself. All three games combined sold 22.75 million copies, almost the total Street Fighter sales, 23.39 million copies, spread over 25 titles.
With sales like that SSBB went much beyond the public for fighting games and it is unclear how many extra copies it would sell if they included better online support.