SomeoneF is right, though I will say I've had plenty of fun with this game on my own as well. Unfortunately, the AI in this game is borderline retarded, so any real challenge where you get to test the decks you'll have made will rest entirely on your luck with playing random online. Many don't actually fight, and as its possible to connect with those in Japan, lag can become another huge issue.
It has more or less become a singler player fighting game, though if you can convince a friend or two, it can still be a great game. The first game, Jump Superstars, had no online at all, and I still had hundreds of hours of fun playing, so its really up to you.
Just because something is inexplicted, doesn't mean its inexplicable.